THE MICROSCOPE. 



and should be made to draw back so as to clear the aper- 

 ture altogether, when the tube A acts as a single micro- 

 scope. When the prism is thrust in, it coDects a portion 

 of the rays, and reflects them to the opposite side of 

 the tube, where an opening is made large enough to allow 

 them to pass through, into the supplementary body, 

 E, which in size corresponds to the main tube ; the re- 

 mainder of the rays pass uninterruptedly up the principal 

 body. 



Tig. 2 is an enlarged outline 

 of the prism. Let E E be a ray 

 of light having passed through 

 the object-glass and entering the 

 prism at right angles at the point 

 F ; passing on, it is intercepted 

 by the surface A B, which being 

 inclined within the angle of total 

 reflection, the ray is reflected 

 towards H, from which point it 

 is again reflected in the direction 

 required. If the prism be cor- 

 rectly made, and of the smallest 

 size possible for admitting the 

 pencil, the difference between the 

 direct and reflected image is 

 scarcely observable; a faulty 

 prism therefore can be easily detected. 



The adjustment for difference of distance between the 

 eyes is effected by means of the draw tubes ; if they are at 

 the utmost limit of proximity when close in, by drawing 

 them out they can be made to suit every position of eye- 

 sight. This is very conveniently done by means of a rack 

 and pinion movement, as shown in Fig. 3. 



The opaque principle of illumination should be used in 

 all cases where possible, as this gives to objects a more 

 natural appearance. The effect upon looking through a 

 binocular microscope for the first time is very striking; 

 many peculiarities are instantly presented to the eye, which, 

 with a single body, would be observed with difficulty. The 



FIG. 2. 



