COMPOUND MICROSCOPE. 



769 



Provision is also made for adding a polarising apparatus. In addition 

 to the four holes mentioned as needed to admit the requisite amount 

 of light, the diaphragm is furnished -with a fifth hole, into which a 

 Nicoi's prism may be screwed, forming the polariser ; the analyser being 

 screwed into the upper part of an adapter previously to its being 

 attached to the body, i. The polariser is mounted on a double tube, so 

 as to be capable of being evolved by turning a large milled head at the 

 bottom. A condensing lens for illuminating opaque objects may be 

 fitted into the hole at the corner of the stage ; it is so arranged that 

 it can be used in any required position or angle. Among the objects 

 often- furnished with the microscope is a plate of selenite, which, 

 if laid under many animal and vegetable structures whUe being ex- 

 amined by polarised light, will cause them to assume beautiful colours. 

 By means of a Binocular microscope objects may be seen in relief 



Very good microscopes for students are made by Smith and Beck in 

 London, and by Nachet and Hartnack in Paris. One of the latter 

 is shown in figure 947. The figure is one-fourth of the real size of 

 the instrument. The body consists of a telescope tube eight inches 

 in length, held by a split tube three inches long. It maybe elevated 

 or depressed by the hand by a cork-screw movement, and this con- 

 stitutes the coarse adjustment. It is attached to a cross-bar and pil- 

 lar, at the lower portion, of the latter of which there is a fine adjust- 

 ment screw. The stage is three inches broad and two and a half 

 inches deep, with a circular diaphragm below it. The base of this 

 portable instrument is loaded with lead so as to give it steadiness. 

 A similar instrument is made by Nachet, in which there is a broader 

 stage and a broader base, as well as a means of inclining the body of the 

 instrument. The following are the magnifying powers, in diameters 

 linear, of Nachet's compound achromatic microscope for students : — * 



As a portable compound microscope is sometimes wanted by a student, 

 Dr. Bennett has given the accompanying figures of one recommended 

 by Gruby of Paris. In fig. 948 the instnmient is shown in its Case, 



* The price of the instrument, with all these powers, is 1 90 francs, exclusive of duty 9»<1 

 carriage ; mthout No. 2 ocular, and No. i objective, it is 150 francs. 



3d 



