An Unusual Microscope by the name 'OTROPIC'

by Mike Dingley, Australia

As a collector of portable / field type microscopes I have recently acquired an unusual microscope in which the objectives are mounted in a row on an arc-shaped metal bar instead of being mounted on a rotating disc as is the usual scenario. Coarse and fine focusing is operated by turning concentric barrels on the microscope tube. In its normal position the instrument stands 240mm high and 290mm when the drawtube is extended.

At the rear of the stage between the stage clips are the words; Made in Germany Otropic Nr. 826. Hermann Wetzer Pfronten / Bayern. This company produced the Otropic microscope from late 1947 till 1952 or 1953 and produced about 2,500 - 3,000 units which were mostly sent to customers in Hong Kong and Rio de Janeiro. It appears that this was the only microscope to be produced by the company as they normally made other instruments.

The limb and round base are metal coated with a crinkle (wrinkle) finish black paint. The ferrous metal base is circular, 75mm in diameter, 16mm thick and has a conical depression in the top surface allowing full movement of the mirror. The limb is attached to the base via two slot headed bolts from recessed holes under the base. The limb is 89mm high and has a slot 17mm wide from the top which ends 7mm from the base and gives it the appearance of twin pillars. The limb is pivoted with trunnions at the top of the pillar so that the tube, objectives, stage, condenser and mirror can all be tilted.

There is a double sided plain/concave mirror 34mm in diameter which is mounted on a swivelling yoke which in turn is fixed to a swinging arm 40mm in length. All surfaces except the mirror are painted black on brass.

The condenser is nickel plated brass and is non-centerable. It has a thread which screws into a collar which forms part of the underside of the stage. Movement is up and down only. There is an iris diaphragm.

The fixed stage is circular, 75mm in diameter, 7mm thick (not including the condenser mount) and is finished in smooth black paint over brass. The slide holders consist of two spring metal stage clips 35mm long. Around the edge of the circumference is a V-shaped groove to allow accessory stages to be secured with locating screws. Under the stage slightly to the front and left (as facing the instrument) is a lever bent at the rear with a notch which acts as a stop to prevent the condenser from being raised too high.

There are two additional stages and are attached to the existing stage by first removing the stage clips and then placing them on top of it using pins and screws to lock them in position. The first one is a centring stage with its own stage clips fitted. It forms a circle, 86mm across with a flat edge at the rear. It has a rim on the underside 8mm deep in which a pin mounted at the front but inside, acts as a locating pin when inserted into the existing stage groove. At the rear, but on either side, are two screws with knurled heads which also fit into the existing stage groove allowing it to be centred.

The other stage is a rectangular graduated mechanical stage. At the front are two screw bolts with knurled heads angled towards each other which are screwed into the V-shaped groove of the existing stage finally to secure it in position. The slide is held flat by means of a wide U-shaped, spring loaded wire. The x and y stages are graduated. The x axis is numbered 0-8 in 0.5 divisions and the y axis is numbered from 0-16 also in 0.5 divisions. Each main division is equal to 1mm of slide movement. Both x and y axis screws are graduated 0-50 with 1 complete turn equivalent to each AB division or 0.5mm on the stage.

The non-standard objectives are 21mm long and 12mm in diameter. The thread is 8mm X 0,5 pitch. When viewed from the front the objectives line up laterally (left to right) to the stage and each objective is selected by pushing one side of the bar laterally until the suitable objective clicks in to position above the light path. The objective bar is curved upwards (arc shaped) and when viewed from either end there is a female dovetail cut on either side near the top which runs along its length.

The objective carrier slides through the tube with a male dovetail cut in to it. A major drawback with this type of objective mounting is that the top of each objective is open to dust and dirt particles.

The four objectives are marked;

1. Rau-Optic Wetzlar 78:1 n.a. 0.90
2. Rau-Optic Wetzlar 45:1 n.a. 0.85
3. Rau-Optic Wetzlar 30:1 n.a. 0.65
4. no marking 18:1 (no n.a. marking)

The tube has a mechanical length of 125mm and appears to made from nickel-plated brass and is fixed to the microscope limb through a collar and set firmly with a knurled screw. Coarse and fine focusing are not carried out by the conventional methods but are controlled by the movement of concentric barrels forming the tube of the microscope. These barrels are threaded and focusing is achieved by rotating the knurled areas which raises or lowers the objectives.

The coarse focusing tube is 40mm long part of which (27mm) is knurled. Each revolution of the coarse tube is equal to about a 2mm rise in the objective. The fine focusing tube is above the coarse focusing tube and is tapered at its apex where it is marked with 100 graduations each being 0.001mm. There is a draw tube which can be raised 50mm above its normal position.

There are six eyepieces present in the case marked 5X, 8X, 10X, 12X, 15X and 20X Periplan. All eyepieces are 15mm in diameter and are therefore smaller than the standard.

The case measures 270mm X 114mm X 100mm and is hinged in the middle along the long axis so that when open both the lid and base rest flat. It is made from wood and covered in a black cloth. On the lid near the right hand corner is the word Otropic. There are two metal clasps to secure the lid, however, only one is complete. The inside is covered in red felt which shows signs of wear and the microscope and accessories are compartmentalised. There is a space where another accessory should be but is missing. The total weight of the case containing all items is 2105g.

The microscope is quite difficult to use for several reasons: Firstly because the instrument is small, lightweight and has a small diameter base the instrument is easily moved when the focusing tubes are turned. This same problem occurs when the objectives are selected as the bar carrying them is a tight fit in the corresponding dovetail. The base of the instrument has to held when focusing and selecting the various objectives.

All of the objectives are achromatic so that the image, when focused in the centre of the field of view is out of focus towards the periphery. With higher magnifications this "out of focus" image gets larger with a reduction in size of the "in focus" circle. However, the objectives are still in good condition providing a clear image especially when using the 5X eyepiece. The quality of the image deteriorates when the higher power eyepieces are used but cellular details can still be seen with reasonable clarity especially when you consider that the instrument was not intended as research quality.

Comments to the author sent via our contacts page quoting page url plus : ('mdingley','')">Mike Dingley

Editor's note: the Microscopy UK and Micscape Editors would like to thank Mike Dingley, who is a new contributor to Micscape magazine, for submitting this original article.


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