THE MICROSCOPE. 



The most simple, and consequently the cheapest description of 

 mounting, is that in which fewest parts are moveahle. The only 

 parts of a compound microscope which are necessarily moveable 

 are those hy which the instrument is focussed, and the object 

 illuminated. The most simple mechanical expedient for effecting 

 the former is a rack and pinion attached either to the body or the 

 stage, and for the latter the suspension of the reflector upon an 

 horizontal axis, so that it can be inclined at any desired angle to- 

 the axis of the body and the stage. 



Whatever be the form or disposition of the stand, it is- 

 essential that the axis of the object-piece should pass through 

 the centre of the stage, and that the reflector should be so set as 

 to be capable of reflecting light in the direction of this axis. The 

 body is generally a straight tube, the axis of the eye-piece and 

 object-piece being in the same straight line. In the case of 

 instruments mounted after the model of Professor Amici, however,, 

 the body consists of a tube having two parts with their axes at 

 right angles, the axis of the object-piece being vertical, while 

 that of the eye-piece is horizontal. In this case, a prism is fixed 

 in the angle of the tube, at an angle of 45 with the axes by 

 which the rays proceeding vertically from the object-piece are 

 reflected horizontally to the eye-piece, on the principle already 

 explained (30). 



75. One of the most simple models for the mounting of a com- 

 pound microscope was contrived by Frauenhoffer so early as 

 1816, long before achromatic lenses were produced. This model, 

 owing to its great simplicity, convenience, and cheapness, is still 

 extensively used for the lower priced instruments, especially by 

 the continental makers. 



The body of the instrument is attached to a vertical pillar, fig. 

 36, p. 49, and its axis is permanently vertical. It is focussed by 

 a rack and pinion, worked by a milled head on the right of the 

 observer. The stage is fixed in its position, and placed on the 

 top of a short tube, in the lower part of which the reflector is- 

 suspended on an horizontal axis, so that it can be placed at any 

 desired obliquity to the axis of the instrument, and thus can 

 always throw a beam of light upwards to the object. One side 

 of this mirror is concave, and the other plane. 



For the illumination of opaque objects, a lens is attached by 

 a jointed arm to the upper part of the pillar, on which the 

 instrument is supported. 



M. Lerebours, of Paris, makes excellent microscopes on this 

 model, with a triple achromatic object-piece and other accessories,, 

 which he sells at the very moderate price of 90 francs (31. 12s.). 

 Several thousands of these have been sold. 

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