American Microscope Makers
other articles

Fasoldt's Patent Microscope

Journal of the Royal Microscopical Society, 1889

Mr. C. Fasoldt, the well-known ruler of lines, has devised the microscope shown in figure 1.

The peculiarities of the construction (1) the combination of the coarse and fine adjustments in one mechanism, which is shown in figure 2, intended to prevent the breaking of objects and injury of objectives through the accidental moving of the tube; (2) the vertical illuminator, in which by a pair of plates opening angularly by the rotation of a cam and a single diaphragm plate, pivoting together or separately in front of a fixed quadrangular aperture, the light can be variously regulated. The glass disc reflector is attached to a bar, which can be withdrawn for cleaning or replacing by turning the milled-head cap in front. It can also be inclined as well as moved out of the field of view by pulling the bar through the milled-head cap, when the disc lies in the piece of tubing on which the cap fits; (3) the changing nose-piece applied below the vertical illuminator, by which the objective can be attached or released by the action of a trigger piece on a sliding tooth, the inner edge of which has the Society screw thread, and presses the screw of the objective against two similar, but fixed teeth opposite; and (4) the fixed stage-ring has a deep groove round the outer edge, in which the upper plate rotates by means of two short pins on the inner edge of an overlapping flange, two diametral slots in the fixed ring enabling the upper plate to be removed.

The combined coarse and fine adjustments are shown in fig. 2. At the back of the body tube slide is fixed a short screw socket, through which a long coarse threaded screw passes, the rotation of the screw causing the socket, and with it the body tube, to move up or down. Near the lower end of the screw is fixed a small pinion with spiral teeth, in which a similar but much larger pinion engages for the coarse adjustment, raising or lowering the body tube somewhat slowly, after the manner of worm wheel and tangent screw mechanism. The screw has a plain cylindrical fitting at each end, by which the small pinion is kept in close contact with the larger one.

Mr. Fasoldt claims for this system of coarse adjustment the impossiblity of any running-down occurring by the accidental concussion of the body tube, as the mechanism remains locked unless set in motion by the milled heads.

For the fine adjustment a long bent lever is applied to the lower end of the coarse adjusting screw, so as to raise it through a space of about 1/8th inch against the downward pressure of a short spiral spring encircling the upper end, the great difference in the size of the pinions permitting this range of motion without disengaging the teeth. The lever is acted upon at the back by a milled-head micrometer screw.

Mr. Fasoldt writes that he uses the illuminator in the following way: "When the microscope is in position and the object on it first find the object with any objective from 2 to 3/4 inches, using either transmitted light or dark field with light through condenser, the latter standing at an angle of about 45 degrees from the stage, and throwing the light directly on the lines, when the latter will give a spectrum. After having them in focus the objective can be changed for a higher homogeneous immersion lens. Set the lamp about 20 inches distant from illuminator, using the sharp edge of the flame, and in horizontal line with opening of illuminator. I use an achromatic lens 2 inch focus as condenser (1 inch in diameter), and put it further away from illuminator opening than focal distance, the opening being open about the thickness of a penny and the light appears on shutters like a cat's eye. After having light in place and the pin in front of illuminator, to which the reflecting glass is attached, standing in an angle of about 45 degrees, you will see only a partially illuminated field, with dark spot in centre; when you have it so, you are ready for work.

The illuminator can be used only on dry mounts. If you do not want to use the illuminator, the reflector can be drawn out by the bar in which the pin is. Then it forms only a single patent nose-piece.

Before putting the light through the illuminator the object should first be brought in focus, using either oblique or central illumination, for lenses of short working distance. The reflector can be set at any angle by turning the milled cap through which the bar passes to which the reflector is fastened. The milled cap is held down by two pins in the cylinder and a groove in the cap into which the pins pass. There are two notches in the cap, which enter into the round groove directly opposite each other. When they are brought in perpendicular position with the illuminator and to where the pins stand, the whole cap can be taken off for the purpose of putting glass in, should one be broken.

For dry lenses, I place the flame lower than the opening and use no condenser, but open the shutters to the fullest extent. You will obtain different results by using the light at longer and shorter distances. For examining blood corpsucles, latter should be mounted on cover glass, and you get the best results by using less light."

Figure 1 Figure 2 Charles Fasoldt other makers

© 1997, 1998, American Artifacts, Taneytown, MD.
Contact: Richard Van Vleck