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In this article, we describe the construction of a very simple low-cost compound microscope. As shown in Figure 1, the microscope we describe is one that just about anyone can build and will produce a magnification of about 75 times. Microscopes may be thought of as very intricate and mysterious instruments but in reality, they are not as complicated as one may think. Building this simple instrument is not only a fun project, it will help you understand how microscopes work. This microscope, which will cost you no more than about a dollar or so to build, is essentially identical to the expensive microscopes that professionals use. Through this project you will gain an appreciation for the need of using corrective optics to reduce the aberrations. Obviously, the performance of this simple microscope cannot be compared with those more expensive professional instruments, which will produce much clearer and brighter images. Nonetheless, it should compare well to the low-cost microscopes that are sold in the toy or hobby shops. It is our experience that so called "toy microscopes" are a real disaster because they commonly give little more than diffuse images or shadows, and can give a young person a bad impression about microscopes consequently causing them to loose interest in these instruments. However, an instrument of suitable quality has the potential of sparking a young person’s interest and opening up a world of discovery to them.

A microscope is essentially formed by two lenses: the objective and the eyepiece which is also referred to as the ocular. The objective forms a magnified image of the specimen and the eyepiece in turn magnifies this image. In another article entitled "From Lenses to Optical Instruments", we explore how lenses and microscopes work, so, if you feel the need to review or learn more about the basics, please consult this article. Other components such as the main tube, the focusing system, the stage, the condenser and the illuminating system complete the microscope. The instrument we present here is called as a compound microscope because it is formed by two main optical components: the objective and the eyepiece. A simple microscope, on the other hand, comprises a single lens, which is essentially a more or less powerful magnifier. The glass-sphere microscope, which we described in another article of our gallery is such a simple microscope

To build the microscope you will need the following materials:

- Four lenses from disposable cameras. ***
- One 1 mm-thick metal tube or a 170 mm-long piece of 24 to 30 mm-diameter plastic tubing with a nominal wall thickness of 2 mm.
- Plastic tubes of suitable diameter to make the eyepiece and the objective (see Figures 7 and 8)
- Plastic or thick cardboard tubes used as couplings
- Square sheet of stiff and opaque plastic 1 x 90 x 90 mm for the rotating diaphragm
- Piece of mirror 40 x 50 mm
- Sheet of brass or stainless steel 0.5 x 30 x 100 mm
- Pine 20 x 140 x 150 mm for pedestal
- Pine 20 x 50 x 440 mm for the upright member and two supports
- Pine 10 x 90 x 120 for the stage
- Pine 10 x 40 x 51 for the mirror
- Four self-tapping screws ¯ 3,5 x 20 mm for the clamp of supports
- Four self-tapping screws ¯ 3,5 x 40 mm for the supports
- Four self-tapping screws ¯ 4 x 50 mm for pedestal and stage
- Two self-tapping screws ¯ 3 x 10 mm for diaphragm and mirror
- Two self-tapping screws ¯ 2 x 10 mm for mirror
- Four adhesive felt diskettes for pedestal
- Black adhesive velvet for the internal wall of the main tube and to enhance the fluidity of the focusing movement.

*** To build the eyepiece and the objective of this microscope, we will use the lenses salvaged from disposable cameras as shown in Figure 2. After all of the pictures frames are taken, these cameras are taken to the photo shop where the film is removed and the body of the camera is discarded. What we need for this project is precisely what the photo shops throw away. So, go to your local photo shop and ask them for at least four disposable cameras. If possible, try to get four identical cameras. If available, you may ask your photo shop for some additional camera bodies and can keep them as a reserve. BEWARE! Do not open cameras with a flash system because you are at risk getting a severe electrical shock. The circuit that feeds the flash yields a very high voltage. For this project you should only use cameras with no flash.

Disassemble these cameras and recover all the lenses you find. Usually, the objective of these cameras is a transparent plastic meniscus. A meniscus is a concave-convex lens. Try to find the focal length written somewhere on the camera body. For these cameras it is usually 35 mm. For our project, we will use the main lenses of these cameras. Put aside the smaller and more powerful lenses that are used to magnify the picture frame numbers. You may want to use these smaller lenses later to see if they can be suitable as objective lens. If needed, cut out it from its base using a socket punch.

While removing the lenses, try not to dirty them. To avoid leaving fingerprints on the lenses, handle them by holding them by their edge. You can also use latex or thin cotton gloves. When you mount them, remove any deposited dust. Blow off any dust then, clean these lenses with a clean and moist cotton cloth. Do not use paper towels because the paper sometimes includes mineral powders that can scratch the surfaces of the lenses. These plastic lenses are very delicate, so try to handle them as little as possible.

The body of the microscope provides support for the different parts of the instrument and gives it stability. The body can be built with small pieces of wood joined with screws. Figure 3 shows the structure of the microscope with the principal dimensions. All of the pieces are fixed to the upright member with two screws. Place four adhesive felt pads under the base of the microscope.

One of the more important parts of a microscope is the body tube. The objective and the eyepiece are mounted at both ends of the tube as shown in Figure 9. The body tube is held in place by two supports through which it passes as illustrated in Figure 4. The body tube can be made of either plastic (2 mm thick) or metal (1 mm thick). For this project we used a rigid piece of plastic water pipe. Avoid cardboard if possible because it will wear out in the long run. The outer diameter of this tube should be between 24 and 30 mm in diameter. Cut a 170 mm length of pipe which you will have to adjust to the measurements given in the Figure 9.

The stage is formed by a piece of wood that has a hole in it to allow light to pass through. A rotating diaphragm is mounted under the stage. To make hole in the stage, first mount it on the upright member and fix the body tube to its supports. Then drop the body tube onto the stage, and with a pencil, draw a circle around the tube. At the center of this circle drill out a hole of about 12 mm in diameter. Blacken the inside of this hole using a black felt-tip pen or India ink.

The next step is to mount a rotating diaphragm under the stage. It is a disk of opaque and rigid plastic 1 mm in thickness, with a series of holes of increasing diameter arranged along a circumference as illustrated in Figure 5. This diaphragm serves to adjust the amount of light that arrives to the specimen. When you assemble it, pay attention to correctly align its holes to the objective. A flat washer should be placed on either side of the disk. Tighten the screw so that it slightly restrains the movement of the disk.

In true objectives plano-convex lenses and special menisci are often used. Several of these menisci are mounted close one another with the plane or concave surface facing the specimen. As indicated in the Figure 8, place the two remaining menisci at about 2 mm from each other by means of a little gap ring. As mentioned previously, it is preferable to make the objective tube of plastic rather than cardboard.

If you use only one of these lenses in the objective, you will obtain a magnification that is about one half of the one described here. Using this principle, you can then make your microscope with objectives of different magnification.

When we first tested the objective and eyepiece described in this article, we barely saw anything. The image was extremely blurred and difficult to focus. Using a piece of dark plastic film scavenged form an old floppy disk, we made a diaphragm with a 1.5 mm diameter aperture and placed it in front of the first lens of the objective. The addition of this diaphragm produced a satisfactory image. With this improvement, we were able to distinguish the small suction cups on the antennas (feelers) of aphids and to observe protists.

The lenses obtained from the disposable cameras are afflicted by strong aberrations when they are used at their full opening. Using glass lenses should improve the quality of the image, but not radically. In fact, to obtain sharper images you should use achromatic lenses. Fortunately, there exists a method to markedly improve the performance of the lenses we are using. All that is needed is to place a diaphragm in front to the objective lenses. This diaphragm limits the aperture of the lenses and makes use of the better part of the lenses. The aperture of this diaphragm depends on the lenses you are using, the power of the objective, its level of correction, etc. Keep in mind that as the diameter of the diaphragm decreases, the quantity of light passing through the objective will also decrease. You will therefore have to use more light to obtain a sufficient brightness to adequately see the image. On the other hand you should not make this aperture too small because the sharpness of the image will start to decrease. Therefore, try different diaphragm diameters until you obtain a suitably sharp image. This aperture has the function of reducing the aberrations of the objective and greatly improves the quality of images observed.

What is the magnification of this microscope ?

You can calculate the magnification of your microscope by means of the optical formulas presented in Table 1. As indicated by the formula no. 6, the magnification of a microscope is given by the product of the power of the objective by that of the eyepiece:

With this relationship, let us calculate the magnification power of the objective and the eyepiece.

MAGNIFICATION POWER OF THE OBJECTIVE
Applying formula 2 of the Table 1 to the objective that we built, we calculate that fob = 18.2 mm
Applying the formula 1 of Table 1, we determine that the distance objective-specimen is p = 20.6 mm
Applying formula 5 of Table 1, as the distance objective-image is q = 160 mm, the power of the objective is Mob = 160/20.6
Mob = 7.77 X

MAGNIFICATION POWER OF THE EYEPIECE
Applying formula 2 of Table 1 to the eyepiece that we built, we determine that fep = 26,06 mm.
And by applying the formula 4 of Table 1, we calculate that Mep = 9.6 X

TOTAL MAGNIFICATION OF THE MICROSCOPE
Applying formula 6 of the Table 1, the magnification of the microscope will be: Mmic = 7.77 x 9.6
Mmic = 74.6 X

There is another empirical method to determine the power of a microscope. Take a ruler with thin and sharp divisions and place it under the objective and focus its image. Place a second ruler at the distance of 250 mm from your eyes. Now, with one eye look through the microscope and with the other eye focused on the second ruler. At this point, superimpose the two images and determine how many divisions of the first ruler seen with the microscope correspond to the second ruler seen with naked eye. We understand that the first time you will try this exercise you may find it rather difficult. Do not get discouraged, with a little practice and perseverance you should succeed. For an amateur microscopist, optical acrobatics of this type are quite normal. Moreover, despite your best efforts to make careful calculations, measurement errors are unavoidable. That is why it is a good idea to double check your calculations using another different empirical method. We present this empirical method as an option for those who enjoy this kind of challenge.

To obtain sharp images, you have to adjust the distance between the objective and the specimen. This operation is referred to as : "focusing". In true microscopes, this adjustment is made by means of mechanisms that is rather complex to build. To focus our little microscope, we use a simple friction coupling that has the advantage of being a simple yet effective mechanism. What is a friction coupling? Simply, the body tube will not be fixed into position, but it will be mounted so it can slide upward or downward with a little force.

As illustrated in Figure 3, the body tube is placed into two adjustable supports. With a set of screws it is possible adjust the force with which the body tube is kept in place by each support. Adjust these screws so that the body tube is tight enough to prevent it from falling downward under its own weight, but loose enough to allow it to be moved up or down by hand.

As described later on, approach a light source to the microscope and adjust the mirror until you see the field brightly and uniformly illuminated. As you have to do with every microscope, place the specimen on a microscope slide, then add a few drops of water, finally cover the specimen with a thin coverslip. Place this specimen on the stage under the objective, with the specimen centered. Adjust the focus and retouch the position of the slide also to explore the different parts of the specimen.

By daylight:
- Set up your microscope near a window from where you can see the sky
- Turn the microscope with the mirror toward the window
- Set the diaphragm to a large aperture
- Orient the mirror until you'll obtain a sufficient and uniformly spread amount of light
- Insert the slide on the stage
- Center the sample by naked eye
- Focus the image
- Replace the diaphragm with another narrower aperture
- Retouch all adjustments until you'll obtain the best conditions for observing
- Never use the direct sun light : you would have too bright, contrasted images lacking of any detail.

By night:
- Approach a lamp with a light with frosted bulb
- Complete the adjustments as described above.

The use of an electric lamp will allow you to obtain better results than with the light of the window, so we suggest to follow this method even by day.

This microscope has the characteristic of being inexpensive and simple to be build. However, if you want to improve on it, please feel free to do so. We indicate in this second part some possible ways that you can improve your microscope. Bear in mind that these modifications, even if they allow you to improve its performance, will render it more complex. On the other hand, why should you limit yourself when innumerable variations and improvements are possible? Besides, why should you limit yourself to the simpler model if you can improve it and if you enjoys yourself while doing so? We are convinced that many readers will enjoy trying different improvements, using lenses or parts they already possess, etc. Moreover, from these experiments they will stand to learn in the process. The following section is intended for those more advanced experimenters who want to perfect their microscope and experiment with different optical and construction solutions. In this section, we provide you with some additional information to guide you in conducting your own experiments.

The best way to improve the performance of this microscope is by improving the optics. Indeed, by using better quality lenses you can improve the performances of this little instrument. In fact, for this project we used plastic lenses, but replacing them with glass lenses you will be able to obtain a substantial improvement. Another means to improve the quality of the images is to use achromatic lenses in the objective. Achromatic lenses are not indispensable to make good quality eyepieces, but plano-convex lenses will do fine. Let us start dealing with the art of amateur eyepieces construction, which is easier than making objectives.

The eyepiece has the main task of magnifying the image formed by the objective and it has to do this by limiting the optical aberrations. There are many eyepiece models, here we'll describe only those that are easier to build. Only two plano-convex lenses are enough to make a high quality eyepiece. Depending on quality of the lenses you manage to obtain, you can a Ramsden or Huygen type eyepiece. They are two particularly simple eyepieces to build which were designed by their authors to minimize optical aberrations. In some cases they are designed to compensate for aberrations produced by the objectives. These models are widely used in modern microscopes and in telescopes. The Huygens eyepiece is probably the most widespread model in use today. If you manage to obtain achromatic doublets of short focal length lenses you can build three other models of eyepieces of still higher quality. Usually a field diaphragm is inserted in the eyepiece mount at the focal point. This diaphragm has also the important function of preventing reflections against the inner surfaces of the eyepiece.

Ramsden eyepiece.
The Ramsden eyepiece is formed by two plano-convex lenses of same focal (fa = fb), with the convex surfaces facing each other (Figure 10). The lens nearest to observer is referred to as the eye lens while the other is designated as the field lens. The distance d between these lenses should be equal to their focal length. This causes the eye lens to focus on any imperfections and dust particles present on the field lens. To avoid this problem the distance between the lenses is reduced to two third of its focal length d = 2/3fa. Unfortunately, this type of eyepiece is not able completely avoid this problem and it provides a quite narrow field. You can also try to keep the two lenses at the distance of one half of the focal length d = fa/2.

Huygens eyepiece.
This eyepeice is formed by two plano-convex or biconvex lenses. Both lenses are oriented with the convex surface toward the objective (see Figure 10). These lenses must to be of different focal lengths. In general, the two focal lengths have to be in the ratio of 1:3 or of 1:2. The distance between the lenses must be equal to half of the sum of the respective focal lengths d = (fa+fb)/2, where fa is the field lens focal length and fb is the eye lens focal length. Let us consider a couple of examples: if fa = 30 mm and fb = 10 mm, the distance of the respective plane surfaces has to be 20 mm. In another example, fa = 30 mm and fb = 15 mm, the distance among the respective plane surfaces should be 22.5 mm. The focus plane of the Huygens eyepiece is placed between the two lenses. Hence, the field diaphragm has to be on the focus plane of the eye lens.

Kellner eyepiece.
This model is derived from the Ramsden eyepiece. It is obtained by replacing the eye lens with an achromatic doublet. With this eyepiece model you should obtain a better chromatic correction and a higher eye distance. You can do the same with the Huygens eyepiece and in this case, as field lens you can use a biconvex lens also. In these eyepieces, the proportions for the eyepieces are the same as those from which they are derived.

Symmetrical eyepiece.
This model is very simple to build and it is formed by two identical achromatic doublets. They have to be kept very close and placed in a symmetrical way. The focal length of this eyepiece is equal to about one half of that of each doublet. It provides excellent performances including good correction of aberrations, a very wide field of view and a high eye distance. Often, this model is called a Plšssl eyepiece, but this is incorrect because the Plšssl eyepiece has a further lens placed in an intermediate position. It is more accurate to call it a symmetrical eyepiece.

We recall that the objective has the role of producing a magnified image of the object you are observing which is further magnified by the eyepiece. Unlike the eyepieces which can be corrected for aberrations without using achromatic lenses, the objectives must be achromatic to produce sharp images.

Just as with prisms, a lens will also bend light to varying degree depending on the color of the light. Because of this phenomenon, a normal lenses will focus the various colors at different distances, thus producing a blurred image as shown in Figure 11. This phenomenon is called chromatic aberration and it is the worst of a several aberrations that can afflict normal lenses. The first microscopists had deal with this problem and for a long time early microscopes, like telescopes, produced blurred images. This problem was resolved when objectives made of two lenses with different indices of refraction were used. These objectives originally were designed in such a way that the chromatic defect produced by the first lens was compensated by the opposite defect produced by the second lens, with the result that the various colors were focused at the same distance thereby producing a sharper image.

Usually, these lenses are cemented together in pairs (doublets) for the correction of the red and blue colors (achromatic lenses) or in groups of three (triplets) to obtain a chromatic correction that is even better for each of the three primary colors: red, green, blue (apochromatic lenses). In other cases they are kept separate.

Objectives are also afflicted by other aberrations, among them are the spherical aberrations which are probably the most important type of aberration after the chromatic aberrations. The planachromatic objectives yield a flat image and are designed for photography. The type of corrections used in these objectives is intermediate between the achromatic and the apochromatic objectives.

With normal lenses (non achromatic) you can obtain fairly good images as long as you limit yourself to moderate magnifications, whereas for high magnifications the use of achromatic lenses in the objectives becomes necessary. For this project, we can use both achromatic or non achromatic lenses. The use of normal lenses demonstrates what chromatic aberrations are and how important it is to eliminate these aberrations to obtain sharp images as the magnification is increased. In general, the use of normal lenses allows you to obtain satisfactory images up to about 100 X, providing you use a diaphragm on the objective.

The objective is the most important part of the microscope. The manufacturers of commercial microscopes design their objectives by means of complex optical calculations and produce lenses according to the parameters which they have defined analytically. Both the design and the manufacturing of objectives are beyond the range of the amateur. However, even if the fabrication of objectives is more complex than that of eyepieces, we will try to fabricate a better objective than the one we have described. In a first step, we will try to obtain the maximum possible performance using normal lenses. Thereafter, we will consider using achromatic lenses.

If you have short focal length plano-convex glass lenses, replace the plastic ones with these glass lenses. Glass lenses are usually of higher quality.

Tube lenght

In manufactured objectives, the first lens is often made of a little plano-convex lens facing the specimen, followed by one or more other lenses. They can be plano-convex, meniscus or achromatic lenses. Normally, in the objectives the lenses are placed with the plane or concave surface toward the specimen. When two equal achromatic doublets are used, they are often placed in a symmetrical arrangement. Other types of objectives follow schemes that are similar to these and the correction of chromatic aberrations is not always made with cemented lenses. Often, the low power objectives are made of a single achromatic doublet.

For the amateur microscope builder, the construction of objectives should follow these principles :
- Do not try to obtain high magnification powers
- Use few lenses as possible
- Use plano-convex lenses or menisci or achromatic doublets
- Place the most powerful lens closest to the specimen (if possible a plano-convex one)
- Keep the plane or concave surfaces turned toward the specimen
- Try to keep all of the lenses centered
- diaphragm the objective to reduce the aberrations
- If possible use an achromatic doublet alone
- If you use two identical achromatic doublets, place them in a symmetrical arrangement and try to keep them at different distances
- Try to use a plano-convex lens followed by an achromatic doublet, or two equal doublets.

Using of achromatic lenses allows you to obtain high quality images and you will not have to place a diaphragm in the objective. Buying a 10X or 20X achromatic objective would save you many problems. In the case where you use an objective for microscope, the mechanical tube length L (normally 160 or 170 mm) should be written on it. As shown in Figure 12, this is the distance between the stop of the objective and that of the eyepiece. Clearly, if you use an achromatic objective for this microscope, you should use a good quality eyepiece also, such as a 10X Huygens eyepiece.

On the market there are also infinity-corrected objectives. In this case, the mechanical tube length is indicated by the symbol for infinity. These objectives are designed to work exactly at the focal distance from the specimen and so they produce the image at the infinity. An intermediate lens, placed in the body tube, focuses this image on the focal plane of the eyepiece. Because of the need for an intermediate lens, the use of the infinity-corrected objectives is a little more complicated than the normal ones. For the sake of simplicity it is better to avoid using this type of objective. If you want more information on these objectives, please refer to the web site that we have indicated in the bibliography.

The following expressions will help you to calculate the focal length and the power of simple eyepieces and objectives, assuming they are systems of thin lenses. All dimensions are expressed in mm. To determine the focal length of these lenses read our article: "From Lenses to Optical Instruments".

Table 1 - Some optical formulas
Terminology	f	focal length of a single lens or system of lenses
	p	objective-object distance
	q	objective-image distance
	f_a	focal length of the lens A (es: the field lens)
	f_b	focal length of the lens B (es: the eye lens)
	f_ab	focal length of the system of two lenses A and B
	d	distance between two thin lenses
	D	distance of the focus plane from the front lens

1	relationship between the focal length and the p and q distances	1/f = 1/p +1/q
2	focal length of a system of two lenses (i.e: the eyepiece)	f_ab = f_af_b/(f_a+f_b-d)
3	distance of the front focus plane from the nearer lens	D = f_ab( f_b-d)/f_b
4	eyepiece magnification power	M_ep = 250/f_ab
5	objective magnification power	M_ob= q/p
6	magnification power of the microscope	M_mic = M_ob x M_ep

As described in the first part of this article, the image is focused by manually moving the body tube by hand. The tube slides on its supports because it is held in place by a moderated force with a braking device. With little effort it is possible add a steel wire focusing mechanism that you can maneuver with knobs as illustrated in Figure 13. The overall cost this improvement will barely cost you more than another dollar or two.

Materials for the focusing device:
- Steel cable for model aircraft construction (0.4 mm-diameter nylon coated steel braided cable. The outside diameter of this cable should be 0.7 mm). You can buy this cable in a model aircraft or hardware store.
- Steel shaft ¿ 10 mm (its surface has to be regular and smooth)

- Two spacers tubes
- L-shaped aluminum bar 2 mm thick to make the two supports for the cable
- Two M 3 x 7 mm screws for the supports of the cable
- Two M 3 nuts to stop the two supports of the cable
- M 3 x 25 mm screw for the solution with adjustable brace
- Two M 3 nuts for the solution with adjustable brace
- Steel wire ¿ 1.2 for the solution with hook (obtain the hook from a clip).

The heart of this focusing device is a thin and flexible steel cable that is fixed to the body tube. This cable is not made of a single wire, but of very thin braided wires that render it more flexible. In fact, it has to wrap around the shaft for three turns. The shaft in turn has to be fixed to a support of the body tube. To do this you can use two little plastic blocks, each kept in position with a screw. As shown in the Figure 14, the cable ends in little holes drilled into two L-shaped aluminum bars that are mounted on the body tube. The lower one it is terminated with a knot whereas the upper end of the cable is tightened with a screw. Figure 14 shows two possible solutions to do this.

The first solution: In the simpler case, depicted on the right side of the Figure 14, the cable is connected to a small hook. Make the stem of the hook pass through the hole in the L-shaped bar. Using a pair of pliers, pull the hook with a force of about 3 Kg. Bend the stem to keep the cable under tension. Try the focusing system: the body tube should be able to move up and down easily and firmly. Check that the shaft does not slip with respect to the cable when it is turned with the knobs. If this occurs, increase the tension of the cable.

The second solution: In the second solution, depicted in the center-left section of the Figure 14, the tension of the cable is tightened using a screw. The tension screw has to limit itself to go up and down without rotating. To accomplish this, the hole through which it passes is not threaded and a nut is used to adjust the tension of the screw. Once the tension of cable is set, the screw is then set in place with a second lock nut. Adjust the tension of the cable until it stop slipping on the shaft.

Before mounting this system make notch of the supports deeper to allow the passage of cable supports. Fasten the two plastic supports of the shaft once the cable is wound and tightened. Try to keep the cable parallel to the body tube.

Despite its odd appearance, this focusing system will work very well. Its movement will be more fluid than a classical and much more complicated mechanism using a dovetail slide and a rack and pinion device.

High quality microscopes are usually equipped with a mechanism to make coarse and fine adjustments to the focus. The coarse adjustment provides a quick but rough focus, while the fine adjustment allows you to adjust the focus more precisely. To equip our little microscope with a fine focusing mechanism, we will use a differential screw as illustrated in the Figure 15. This mechanism is a simple but effective system, made up of two coaxial screws of different pitch. When the screws are rotated counterclockwise, the larger screw will come out from its nut a greater distance than the smaller screw, which will move into its nut. In this way the stage will be pushed upwards by a distance equal to the difference of in pitch of the two screws.

To make this fine focusing mechanism, you can use a 3 mm and a 5 mm machine screw. First, you must cut away their heads then you will have to join them end-to-end. To join the two screws, insert the end of each screw into a short brass or steel sleeve and solder them together. With washers and nuts, fix a cap from a tube of toothpaste on the middle portions of the differential screw. Drill two aligned holes, one under the stage and the other on the base of the microscope. With a vise, pressure the two nuts into these holes, one for each screw then mount the differential screw in place.

Because the thread of one screw has a pitch of 0.8 mm while the thread of the second has a pitch of 0.5 mm, a complete rotation of the differential screw will produce a shift of 0.3 mm of the stage. This system should not be used for more than one turn. You should also make sure that the stage is always pushed upward when adjusting the focus. With the coarse adjustment try to get best focus possible then sharpen up the image with the fine adjustment.

The role of the condenser is to concentrate the light on the specimen. This is important because as the microscope power increases, the quantity of light needed to illuminate the object increases too.

Converging lens and rotating diaphragm
For this simple microscope, a condenser is not indispensable, but your observations could benefit from it. To install a condenser, you can insert a strong plano-convex lens under the stage, in line with the hole in the stage. This lens should have a focal length between 10 and 25 mm and must have the plane surface turned upward. In this way you will have a higher quantity of light and with a more suitable convergence to produce good images. If you can obtain better optical components for this instrument, you could add a second powerful biconvex lens under the first one as illustrated in Figure 15.

Usually, microscope condensers are provided with an iris diaphragm, where the aperture is continuously adjustable to fit the cone of light that is directed to the specimen according to the numerical aperture of the objective. An iris diaphragm is rather expensive for this simple microscope and making one would be a rather laborious undertaking. However, the rotating diaphragm we described will work well even if it will not be very sophisticated. If you succeed in obtaining an iris diaphragm, place it under the condenser.

Adjusting the condenser diaphragm
With the diaphragm at its maximum aperture you will see well defined images that are also extremely pale with little details. By narrowing the aperture of the diaphragm, the image will become more contrasted and the outline will become more pronounced. Moreover, the depth of field will increase. Continuing to narrow the diaphragm aperture, the outline of the objects will fuse and the sharpness of the image will degrade. So, by adjusting the aperture you can try to obtain a balance between sharpness and contrast of the image.

This section describes approaches to produce a source of light to the microscope. Different solutions are possible:

Mirror and window
Approach a window and direct some light on the specimen with the mirror. This is the simplest solution, adopted also by student microscopes with achromatic objectives.

Mirror and lamp with frosted bulb
If you try to use a fluorescent tube as light source, you will notice that only the lines laid out in one direction will be focused, while the others will be diffuse. This is due to the long and narrow shape of the light source. Windows also often have irregular shapes that have a similar effect. Circular and uniformly bright sources of light are necessary for the microscopes. To illuminate your microscope you could use an adjustable lamp with a frosted light bulb placed at the distance of about 30 cm from the mirror.

Lamp and diffuser
Using the mirror, once you have found a suitable light source, you'll not able to move the microscope any more without altering the illumination conditions you have established. To avoid this problem you can make a closed box containing a lamp with a circular hole on its lid. The hole has to be covered by a diffuser of frosted glass so that the filament of the lamp be not visible. To set up this illuminator it will be necessary to conduct some tests so to determine the suitable diameter of the hole and its distance from the microscope. It is also useful to provide it with diaphragms of different aperture and with a blue filter to raise the color temperature of the filament lamp. With a little effort you can make the intensity of the light adjustable. In building this illuminator, use a low voltage lamp, materials which do not conduct electricity and are heat resistent.

Illumination with lamp, lenses and mirror - Kšhler illumination
In common microscopes, the light source is formed by a lamp with a bare special filament. According to Kšhler's principles of illumination, broadly adopted in the modern microscopy, some lenses and a mirror are used to bring the image of the filament onto the aperture of the diaphragm of the condenser. In this way, an intense and homogeneous bundle of light is obtained.

To achieve the best results, it is necessary that the light arrives at the specimen with a convergence that is close to when it enters the objective. To achieve this, adjustments are made to the aperture of the diaphragm which is usually placed under the condenser. In these illuminating systems, close to the lamp, there is also a field diaphragm which can limit the illuminated zone to a little circular area in the center of the observation field. There are also filters, for example the blue filter which compensates the color temperature of the lamp.

The last illumination system we have described is commonly used with commercial microscopes. To implement such a system for this microscope project would probably not be justified.

We have already described how you can obtain the lenses needed to build a basic microscope. Further on we will provide you with some information on how to obtain lenses suitable to improve the optics for this project. Amateur scientists or hobbyists often have collections of various lenses, obtained from different sources and know where to get those specific lenses for a particular application. Nevertheless, if you do not have suitable lenses to improve your instrument, here are some suggestions where you can look for them:
- Optical and photography stores
- Photographic, astronomy, minerals, electronics fairs where you can find retailers of lenses and of new or used optical instruments
- Car wreckers and scrap yards. Here you can get very strong lenses that are about 50 mm diameter, which are used in headlamps of some model of car. One of these lenses can be used as a second lens of the condenser
- Numerous mail order houses selling optical products are also found on Internet.

The Edmund Scientific company is well known in the field of scientific equipment for industry, schools and amateurs. This company also sells low cost lens kits. They are available in sets of 10 or 20 or 30 lenses suitable for use in simple instruments like telescopes and microscopes. You may be able to find the lenses that are best suited to build your eyepieces and objectives. This company sells also iris diaphragms.

Many other companies produce or sell lenses. Some of them are present on the Net. To find them, with a search engine, look for:

The terms: -fiber -laser serve to avoid the great deal of fiber optic and laser kits and which don't serve you (the "-" sign indicates to the search engine to exclude the documents which own the word which follows it).

As for the observations you can make with this microscope, we refer you to what we have already presented in the article entitled: "Glass-Sphere Microscope"

Although the little basic instrument described in this article is simple and costs less than about a dollar to build, it should work well and will give you an appreciation of how the more expensive professional microscopes work. With this simple project, the amateur scientist can gain an understanding of the principles that make microscopes work. If you are so inclined to carry this project further, you can experiment with different mechanical improvements, different positions of the lenses, additional corrective optics, or integrate your instrument with devices that can improve its performance. This project should not be seen as "closed" project that must be made according the precise specifications, but it is opened to a number of developments. We are convinced that these possible developments will be specially welcomed by all those who feel the need of spaces for their creativity and who possess curiosity toward science and nature.

Would you like to know more about microscopes? Here are some resources available on the internet: The first site, The Molecular Expressions Website, is a gold mine of information about microscopy. This is an example of what a good Internet site should be. While this site is mostly for users of professional microscopes than amateurs who build their own instruments, it has a lot of information that should be useful to amateurs. The Encyclopedia Britannica will also provide you with considerable interesting information about microscopes.