The designs of microscopes have changed rather dramatically during the past century. The ones made before World War II are the "classical" microscopes. The best ones were very well constructed because they were hand crafted with "tender loving care." However, their design was rather primitive. The design of microscopes changed dramatically starting about 1950. Many microscopes made in the 1960s and 1970s were still produced in the old tradition of careful craftmanship. Extraordinary care and precision using very high quality materials was the rule of the day. These instruments should easily be in good condition after a century of use! Wild M40 microscopes are certainly in this class.

Few—if any

Lenses, especially objectives, follow a very different pattern. The development of powerful computers and new types of glass has resulted in modern objectives being dramatically superior to those produced in the past. Lenses produced by even the best lens makers in 1950 are dreadful when compared to lenses made more recently. The apochromats and fluorites of 1950 had beautiful resolution in their centres, but they had extreme curvature of field. The achromats had flatter fields, but they had poorer resolution.

The dramatic improvement in lens quality seems to have occurred around 1970. Modern lenses are able to achieve flat fields with little chromatic and spherical aberrations. Modern apochromats are corrected for chromatic aberration at four or five wavelengths. The flat field ones provide a flat field across wide field oculars.

Some time ago I obtained a Wild M40 microscope through an eBay auction. It was missing many parts, but was in overall excellent condition. It lacked both a bulb and a power supply. In the February 2006 Micscape, I described how I machined a special holder for the so called "Star LEDs" to provide a superior illumination system for this microscope. When I originally purchased this instrument it was my intention to make it into a really first class instrument. The goal was not to bring it back to its original condition so that it would be museum piece, but rather the goal was to bring it up to modern optical standards.

When I obtained this M40 it had three objectives designed for the 38mm parfocal standard. All were Wild, a 4X, a 10X phase, and a 20X phase for 1.0mm thick glass. Shortly after the era of this instrument most makers started adhering to the DIN standard of 45mm distance between the objective threads and the object plane. The design of the Wild M40 demands that all lenses be near par focal to prevent damaging them by accidently crushing one during focusing! As soon as I had obtained this instrument, I started looking for lenses. After searching the Internet and eBay auctions for nearly a year I was able to obtain the following set:

• 1. Zeiss 2.5x flatfield DIN.
• 2. Unbranded Chinese Bright Contrast Phase 10X, 0.25 na flatfield
• 3. Olympus LWDCDplan 20x 0.40 na 0-2 mm adjustable phase
• 4. Olympus LWDCDplan 40x 0.55 na 0-2 mm adjustable phase
• 5. Unbranded Chinese 10x 0.25 na flatfield, strain free, not phase
• 6. Unbranded Chinese 25x 0.40 na LWD flatfield, strain free, not phase

All six of these lenses require compensating eyepieces. It is extremely inconvenient to have a microscope objective set in which some require these and others do not!

I considered it most important to have good 20X and 40X lenses with spherical correction. The two Olympus lenses are truly excellent.

"Unbranded" Chinese lenses have a very bad reputation. The reality seems to be that purchasing one is rather like going to Monte Carlo or Las Vegas! These lenses vary from extraordinarily good to extraordinarily bad. The ones in the above set are examples of fairly good lenses of this sort. They have quite flat fields, and have good general correction.

Bringing this instrument into working condition required much more than purchasing objectives! Before they could be installed safely there was an extremely important and truly urgent modification that had to be completed!

The original instrument was missing the high numerical aperture phase ring, it did have the one that corresponds to the Wild 10X objective. The Wild M40 use a metal cylinders with one end holding a circular glass disk with the phase ring painted on the glass. The paint was peeling from the one that came with the microscope, and it was not the correct size for ANY of the three phase objectives in the new collection. Thus it was time to obtain phase rings. Since no one apparently manufactures these for this microscope there is only one source for them—they have to be machined!

It seemed to me that the microscope manufacturers' system of painting rings on glass was an innately poor design. It really makes more sense to have metal rings. Paint tends to peel from glass after several years. Attaching metal rings with silicone cement should obviously make much more durable rings.

The design of one for the 40X was a bit difficult. The phase ring needs to be so large that there is no easy way to attach a glass mount to hold it. It turned out that schedule 40 "one inch" pipe has the perfect diameter for the outside of a phase ring for the 40X Olympus objective. (Oddly enough, schedule 40 pipe has neither its inner or outside diameter equal to 1.00 inch!) I obtained a piece of brass schedule 40 pipe and used it for this phase ring.

The construction details follow:

• 1. Using a band saw cut off a piece of brass schedule 40 pipe having at least 3 or 4mm larger diameter than the holder in which it is to fit so that it is 2 or 3 millimetres longer than stock Wild phase rings.
• 2. Face the ends in a lathe to match the exact length of the original phase ring holder.
• 3. Turn down the rod to the exact diameter that allows it to fit into the holder. One MUST leave it larger on the upper end to be able to remove it easily. The wide upper end should be about 3 or 4 mm high, too much will interfere with the filter holder. This diameter should be machined carefully and to rather high precision—it must fit correctly! (29.96 mm is the perfect diameter.)
• 4. Tape various sizes of coins to the end of the piece and find one that will produce a perfect ring to match the phase ring in the objective. Measure the diameter of the coin.
• 5. Take a piece of metal stock and turn it to the diameter of the coin, and saw off a piece about 4.5 to 5mm thick. Aluminium is preferable for this disk because it has a much lower density.
• 6. Place the metal disk in a drill press on end and bore a hole about 8mm deep toward the centre with a special bit for 3mm taps. Then tap the hole with a 3mm tap.
• 7. Rotate the disk exactly 120 degrees. (Measure it carefully!) Then drill and tap a second hold there like the first.
• 8. Take a long 3mm screw and cut off two pieces about 6 mm long. Cut a slot on one end to turn with a screw driver. Screw these into the metal disk.
• 9. Carefully adjust the two screws on the disk so that when they are placed inside the metal holder the disk is at the exact centre.
• 10. Drill a hole for 3mm tap near the bottom of the bras piece turned above. Tap it, and insert a screw. Tighten it down setting it 120 degrees away from each of the screws in the disk. Check that the disk is still in the centre. Adjust if necessary.
• 11. Remove the screw and cut it off at the exact length so that it would be flush with the surface of the turned surface, cut a slot in it too to adjust it. Put it back into the piece, tighten it, and check again, adjusting again if necessary.

Unfortunately the alloy used for brass schedule 40 pipe is difficult to machine. It tends to stick to tools and this results in a very poor finish. I brought the diameter down to 30.1 mm with normal lathe tools and then used a flat file and sandpaper to bring it down the correct diameter.

This phase ring for the 40x objective functioned perfectly after it was finished.

The design of phase ring described above is certainly not appropriate for the much smaller phase rings needed for the 10X and 20X objectives.

The best technique to make these phase rings seemed to be to mount circular glass disks inside turned metal cylinders and then use silicone cement to attach metal disks to the centres of the glass disks. Because machinable brass is so easy to machine it seemed the only sensible choice for this.

The image below shows one of the phase rings made this way. The best way to make them seems to be the following:

• 1. Determine the dimensions carefully and make a drawing of the final ring.
• 2. Cut off a piece of brass bar stock that is at least 32mm in diameter to about 5 millimetres over twice the diameter required for the phase ring—it is easier to make two than one!
• 3. Place the bar stock in the lathe face and centre drill both ends.
• 4. Turn down down both ends of the rod to 29.96 mm for exactly 16mm.
• 5. Knurl the space between the 29.96mm diameters and then cut the piece in half with a band saw right in the middle of the knurling.
• 6. Face the end that was cut off with the band saw and drill a hole all the way through both pieces. This hole must have inside diameter correct for the phase ring needed. Care must be taken not to drill this hole too large or the piece will be ruined.
• 7. Using a boring tool bore out the piece from the knurled end down to about 2 mm of the end. One cannot use a drill bit, because the bottom of this hole must be flat. Its diameter must be enough to be able to drop a 22mm cover slip inside.
• 8. Find or machine a thin metal disk the diameter of the stop. Glue this to the exact centre of a 22mm round cover slip with silicone cement. Use only a bit, so that none gets onto the cover slip except under the metal disk. The placement of this and its diameter are very critical. Allow the material to cure completely. Place the cover slip with the metal up inside the machined piece. Be sure that it is exactly in the centre, and that the ring of light is even all the way around. Then using a toothpick or wooden match place three small drops of silicone cement against the wall of the tube so that it contacts the edge only of the cover slip. Let it cure. The phase ring is complete. (One could blacken the brass, there are several ways to do this. However reflexions do not seem to be a problem.)

These phase rings also work excellently. The one problem that they have is that cover slips are thin, and these are thus a bit fragile. However, should one break, it would only take a few minutes to repair.

Because phase objectives work best with green illumination I made a second LED holder identical to the one that I described in the February 2006 issue of Micscape, except it has a green LED instead of white. This avoids having to use a green filters. A green LED is much brighter than filtered green light from a white one.

Three of the lenses on the turret are not phase. The condenser does not have an iris. Although it would probably be possible to design some sort of iris system, it seemed simpler to use interchangeable stops. I machined two pieces just like the ones described above, one with a 6mm hole in the centre and the other with a 15mm one to use as stops. The former seems ideal for the 4X lens, the latter for the 10x one. The 25X one works works quite well without any kind of stop in the condenser, but it can also be used with the 15mm diameter stop to provide an image with greater depth of field. Using the 6mm diameter stop with the 40x results in too much loss of resolution, however.

Since the goal has been to keep this instrument in the category of first rate instruments, it needed to have brass body caps. I made a set of these for it using a lathe. Although the instrument originally lacked many important parts, it did have the Wild photo attachment. This was also fitted with a brass body cap!

The original slide holder was not designed for slides and for containers the size of slides. I first machined a little carrier to fit onto the original carrier to hold ordinary slides. Eventually, however, I decided that this was an inferior system, so I machined a complete new carrier for it from aluminium plate. I originally had planned to put clips on it to hold the slides in place, but decided that they really were superfluous.

Actually the special carrier that I made to fit unto the original slide carrier is useful and convenient when using large containers and slides at the same session, otherwise the replacement carrier is far superior.

I also machined a more complex heated stage that uses a small cartridge heater. Using it requires a 10mm objective extension which I obtained from Rolyn optics. I have a LOMO DIN 10X objective that I always use for this purpose because it has a long working distance. Before installing the heated stage I simply remove one of the objectives and install the LOMO 10X objective with the extension. Instead of placing samples on slides, this stage requires placing them between two 18mm cover slips.

Finally there was the matter of oculars. The instrument came without any! For some peculiar reason used oculars sell for rather high prices. Furthermore, such oculars have often been badly damaged from misuse. As mentioned earlier all of the objectives in the torrent require compensating oculars. New Lomo 10x18 compensating oculars sell for less than many used ones! Thus, they were the choice. Their quality is excellent. Although microscope manufacturers have always contended that one must use their brand of compensating oculars with their objectives, generally they are interchangeable.

For a phase telescope I machined a collar from aluminium stock and mounted a old 5x ocular without its eye lens on one end, and a 20x one on the other side. I drilled a 3mm hole in a metal disk for a field stop. I basically followed Paul James' design that he described in June 2005 Micscape.

It is a pleasure to use this wonderful microscope now. Because the objectives come from more than one manufacturer they are not perfectly parfocal. That is not really much of an inconvenience, because there is a substantial "setup" when changing objectives anyway. The phase rings need to be changed, and checks need to be made to be certain that the changed one is properly centred and focused. Turning the fine focus knob a bit is really a very minor thing!

The 2.5X Zeiss flat field objective is very useful. When used with the large phase ring it provides very good dark field illuminated images. When used with the 6mm or 15mm stop it provides excellent bright field images. It is also very useful for locating features to examine at higher power.

The images with all six lenses are quite exceptional. Unlike the original lenses all six give wide flat fields. The wonderful mechanical quality of the old Wild M40 body is now coupled with the superior quality of modern lenses!

I remember using one of these instruments when I was a new faculty member at the University of the Virgin Islands. They were one of the most expensive microscopes available at that time, and they were considered to be at the very top of the line! When it comes to construction quality they still are! The machining and quality construction have resulted in this instrument still retaining a practically perfect mechanical state even though it is now around 40 years old. Back then it really seemed to everyone to be something very special. To me and many other people it still is!

All comments to the author Robert Pavlis are welcomed.

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