116 THE MICROSCOPE. 



enable us to bring tliem out in greater perfection and 

 beauty. Another advantage gained by the last improvement 

 is, that the ordinary single-bodied microscope can be con- 

 verted into a binocular instrument by simply fixing an- 

 other tube at the proper angle, and adding a small prism 

 mounted in a brass box. The latter is made to slide into 

 the lower part of the body immediately over the objective. 

 By its aid the rays of light proceeding from the object 

 are reflected in two directions, which, by means of the 

 double body, are conveyed to both eyes, and thereby a 

 stereoscopic view of the object under observation is ob- 

 tained. The most important point to be observed, when 

 using the binocular, is that each eye has a clear view of 

 the object. This is readily ascertained by closing the eyes 

 alternately without moving the head away from the in- 

 strument, when, if it be found that the two images do not 

 quite coincide, it must be corrected by racking out or in 

 the draw-tubes, which should form a part of the bodies 

 of all binoculars. If both fields be not equally illuminated, 

 the object is not rendered stereoscopic. 



Mr. Wenham's most important improvement consists in 

 splitting up and dividing the pencil of rays proceeding from 

 the objective by the interposition of a prism of the form 

 shown in fig. 67. This is so placed in the body or tube 

 of the microscope (fig. 68, a) as only to interri^^ne-half 

 (a c) of the pencil, the other half (a h) goinllj^ con- 

 tinuously to the field-glass, eye-piece, of the ^micipal 

 body. The interrupted half of the pencil, on its entrance 

 into the prism, is subjected to very slight refraction, since 

 its axial ray is perpendicular to the surface it meets. 

 Within the prism it is subjected to two reflections at 

 h and c, which send it forth again obliquely on the line 

 h towards the eye-piece of the secondary body, to the left- 

 hand side of the figure ; and since at its emergence its 

 axial ray is again perpendicular to the surface of the glass, 

 it suff'ers no more refraction on passing out of the prism 

 than on entering it. By this arrangement, the image re- 

 ceived by the right eye is formed by the rays which have 

 passed through the left half of the objective ; whilst the 

 image received by the left eye is formed by the rays which 

 have passed through the right half, and which have been 



