30 



THE MICROSCOPE AND ITS REVELATIONS. 



between the axes of the eyes in different individuals, is made by draw- 

 ing-out or pushing-in the eye- pieces, which are moved consentaneously 



FIG. 20 



FIG. 



Fro. 19. 



Wenham's Prism. 



Wenham's Stereoscopic Binocular Microscope. 



by means of a milled-head, as shown in Fig. 21. Now, although it may 

 be objected to Mr. Wenham's method (1), that as the rays which pass 

 through the prism and are obliquely reflected into the secondary body, 

 traverse a longer distance than those which pass-on uninterrupedly into 

 the principal body, the picture formed by them will be somewhat larger 

 than that which is formed by the other set; and (2), that the picture 

 formed by the rays which have been subjected to the action of the prism 

 must be inferior in distinctness to that formed by the uninterrupted half 

 of the cone of rays, these objections are found to have no practicable 

 weight. For it is well known to those who have experimented upon the 

 phenomena of Stereoscopic vision (1), that a slight difference in the size 

 of the two pictures is no bar to their perfect combination; and (2), that 

 if one of the pictures be good, the full effect of relief is given to the 

 image, even though the other picture be faint and imperfect, provided 

 that the outlines of latter are sufficiently distinct to represent its perspec- 

 tive projection. Hence if, instead of the two equally half-good pictures- 

 which are obtainable by MM. Nachet's original construction, we had in 

 Mr. Wenham's one good and one indifferent picture, the latter would be 

 decidedly preferable. But, in point of fact, the deterioration of the sec- 

 ond picture in Mr. Wenham's arrangement is less considerable than that 

 of both pictures in the original arrangement of MM. Nachet; so that the 

 optical performance of the Wenham Binocular is in every way superior. 

 It has, in addition, these further advantages over the preceding, First, 

 the greater comfort in using it (especially for some length of time to- 

 gether), which results from the convergence of the axes of the eyes at 

 their usual angle for moderately-near objects; second, that this Binocu- 

 lar arrangement does not necessitate a special instrument, but may be 



