2 Objects for the Microscope. 



square hole is cut, through which the prism c is made 

 to slide so far that its edge will just reach the central 

 line of the objective, and should be made to draw back t 

 so as to clear the aperture altogether, when the tube ' 

 A acts as a single microscope. When the prism is thrust 

 in, it collects 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 sup- 

 plementary body, E, which in size corresponds to the main 

 tube ; the remainder of the rays pass uninterruptedly up 

 the principal body. 



Fig. 2 is an enlarged outline of 

 the prism. Let E E be a ray of light 

 having passed through the object 

 f 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 

 towards H, from which point it is 

 again reflected in the direction re- 

 quired. If the prism be correctly 

 made, and of the smallest size pos- 

 sible for admitting the pencil, the 

 difference between the direct and 

 reflected image is scarcely observable; 

 FIG. 2. a faulty prism can therefore 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, 



