CLEANING AND ADJUSTING THE OPEN UNIVERSITY
McARTHUR MICROSCOPE:
NOTES AND RECOMMENDATIONS

by Guido Santacana, Puerto Rico

 

About two years ago I received an OU plastic McArthur Microscope from Mr. Brian Adams in the UK. This was a microscope that he had cleaned and readjusted. I have been so impressed by the performance of this unit that when he recently announced an OU microscope sale on the Microscopy UK web-site I went ahead and bought several more.They are sold in pairs and I have found that both can be refurbished. The ones on sale have not been restored by Mr. Adams but he will send well written instructions on how to clean and adjust the microscopes. Using his instructions, I have refurbished my microscopes and at the same time have gained some insight into the process. Here is an account of my experiences in working with these microscopes and some photos of the various procedures as they were carried out.

 

Open University (UK) students microscope based on the McArthur design. (Image by Dave Walker)

1) fixed eyepiece
2) selector for external or internal battery lighting
3) light port (for daylight viewing)
4) slide holder (slide sits upside down)
objectives are below slide in case
5) slits for filters eg polarisers
6) focusing knob

 

The first thing to have in mind is the correct tools, specially the screwdriver for the slotted nuts that hold the three sections of the microscope together. The blade of the screwdriver must have at least 5mm in width. In the center of the blade you must file a 2mm slot. I did this with a Dremel tool but a good file and some elbow grease should do just as well. Fig.1 (right) shows the results on my small screwdriver. You may have to play a bit with the file until a near perfect fit is found. It is very important to have a good fit into the slotted nut to be able to hold it firmly for removal.

 

The next step is removal of the 4 slotted nuts. These are located in the bottom part of the microscope toward the back and well covered with white lacquer in some microscopes. The lacquer must be carefully removed by gentle scraping with a pointed tool being careful not to scratch the bottom of the microscope. When the slotted nut is reached, clean the slots out of all the lacquer for a good fit of your prepared screwdriver. Fig.2 (above) shows the bottom of the microscopes with the slotted nuts exposed. Now place a cloth in your working table and turn the microscope with the bottom up. Grasp firmly with your hand and insert the prepared screwdriver in the first slotted nut. Make sure that there is a good fit. If not, go back and work on the screwdriver again until it fits. If the fitting is good, press a bit with the screwdriver and turn it firmly. The slotted nut should start to move with some difficulty at first and then much easier. Go slowly but firmly. Remove the four slotted nuts.

 


Figure 3

 

The three parts that form the microscope body are now loose. These parts are (see Fig.3 above), the top section with the eyepiece, the center section with the lower eyepiece lens and the lower section with the focusing mechanism, objectives and mirrors. Now grasp the microscope firmly at the bottom with one hand and the two upper parts with the other. Carefully separate the bottom section from the top ones. You may have to move the upper parts a bit while sliding them out of the two long screws. See Fig.3 to see what happens as you separate them. Don’t force them, just go about it slowly. After the separation you will have the bottom section that contains the objective and mirrors and the top and middle sections with the light unit, eyepiece and condenser as shown in Fig.3. Now place the bottom section in a safe place.

 


Figure 4

 

Take the top and middle sections and separate the middle section from the top one by sliding it out of the long screws. The middle section contains the bottom lens of the eyepiece. Put it aside for a moment. In the end opposite the eyepiece in the top section you will find the condenser lens Fig.4 (above).

You are now ready to start cleaning. Use a bit of mild household cleaner to clean the plastic body in each section. For the lenses use a 50/50 mixture of isopropyl alcohol and distilled water. Small cotton tipped applicators can be used to clean the lenses. Start with the eyepiece (Fig 5 right). Wet a cotton tipped applicator with the 50/50 alcohol solution and gently pass it over the surface of the lens on both sides. Dry with another applicator but very gently. Clean the top lens and then the bottom lens of the eyepiece (in the middle section).If the lens in the middle section is held by a black plastic ring, you may remove the ring and take the lens out for cleaning. Just make sure that when you put it back the flat face of the lens looks up toward the eyepiece.

Now check the condenser lens. If it is firmly in place clean it as you did with the eyepiece. If it’s loose, then gently separate both sections of the condenser holder by removing the four screws and then two small ones that you will find as the plastic frame that holds the plastic slide holding clips is lifted. Be careful so that the lens don’t fall off. Take the lens out and clean with the alcohol solution (Fig 4). This lens has square corners. You will have to carefully place some nail polish in each corner and center the lens in the top part of its mount (the one with the bigger hole).

Now place the bottom part of the condenser mount in place and replace the whole section on the top section of the microscope using the reverse procedure for removing it. Since now all the lenses in the top section are clean, take the top and middle sections and slide them back together. Refer to the figures above. Place this section in a safe place and take the bottom section of the microscope that contains the objectives and mirrors.

Remove the focusing knob by unscrewing it completely (Fig 6 right). Now take the objective carrier (Fig 7 below) and push it out once, twice and a final and third time in which it should slide out of its mount. You may have to push a bit firmly at the end. Place the objective carrier in a safe place. Turn the bottom part and remove the two screws at the end near the focusing knob (see Fig 6). Separate the two parts that form the bottom section (see Fig.8 below). Now you can see the main part of the focusing mechanism. Look carefully for two springs at the top of the objective mount in Fig 8. You must be careful not to loose them in the next step.

 


Figure 7

 

Remove the four screws that hold the objective mount (be careful with the springs!). Cut a small rectangle from a post card and fit it in the place were the two front screws of the objective carrier go See Fig. Replace the objective carrier being careful to place the springs in their correct hole. Now the bottom of the carrier should be almost flush with the white plastic border. This will return the focusing mechanism of the microscope to normal. The mirrors in the bottom part of this section (Fig. 8) should be cleaned just by blowing some air on them. If a mirror is loose, just refit it in by applying some nail polish to the back and getting it carefully back in its place. Try not to touch the mirror surface.

 


Figure 8

 

Take the objective lens carrier and clean each objective lens with the cotton tipped applicators and alcohol solution. You don’t have to remove the lenses from the carrier to do this. Just go first on one side and then the other. Replace the objective lens carrier into its mount in the bottom section by gently pushing it in place. Now put together the two parts that form the bottom section and make sure that the plastic strips that go over the hole left by removing the focusing knob are back in place (see Fig.8). Replace the front screws that hold the bottom section together.The bottom section is now ready.

 


Figure 9

 

Take the bottom section and align it with the four screws holding up the upper sections (Fig 9 above). Slide the bottom section into the four screws and make sure that the three sections attach together well. Replace the slotted nuts. Screw back the focusing knob. It should screw back easily. The microscope is now ready to be tested. Place a slide on the stage with the cover slip looking down. Focus and look at the image. There should be a clear image at both low and high powers. In some cases the microscope will be almost parfocal. This means that when you change from low to high power, the focus will be retained. Some problem areas that I have encountered include:

Loose mirror - A loose mirror can be easily refitted in its place but the exact position may be tricky. It will be in the correct place if, when you look at the microscope field, there is no sign of a black crescent anywhere. If a black crescent is observed you need to refit the mirror in a lower or upper position. This is the reason for using a mild cement like nail polish. Don’t use a permanent cement like epoxy or cyanoacrylates!!

Plastic strips - There are two little plastic strips where the focusing screw protrudes inside the lower section. Make sure you don’t lose them!

Hard to move objective carrier - This problem is usually solved by cleaning the objective carrier mount with a bit of household or plastic cleaner or by checking if any of the objectives protrudes too much from the bottom of the carrier. Old cement over an objective lens- I found this in one of the microscope objectives. It was easy to remove with a bit of Xylol in a cotton tipped applicator. Make sure that you don’t go over the plastic with the Xylol.

Misaligned objectives - This requires a very careful procedure. Contact me if this is the case.

In the end you will have two excellent and very portable microscopes. They are also great as a first microscope for an inquisitive young mind. Please feel free to contact me for any doubts. I’ll be glad to help.

Comments and queries to Comments to the author sent via our contacts page quoting page url plus : ('gsantacana','')">Guido Santacana welcomed.

 

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