OPTICAL PRINCIPLES OF THE MICROSCOPE. 



35 



tance between the eves was made by giving f a horizontal traversing 

 motion to the prism B and the secondary body placed above it, by means 

 of a screw action. But this method was open to the two objections that 

 the focal distance of the secondary body was thereby 

 altered, and that the traversing fittings were liable 

 to become loose by wear. To meet these, M. Xachet 

 devised the construction represented in Fig. 28; in 

 which the adjustment of the distance between the 

 eye-pieces is effected by altering the angle of conver- 

 gence between the bodies. This is done by turning 

 the screw v, which is furnished with two threads of 

 different speeds, whereby an inclination is given to 

 the prism equal to half the angular displacement 

 of the tube; an arrangement necessitated by the fact 

 that the displacement of the rays reflected by a rotat- 

 ing surface is double the angle described by that 

 surface. 1 As an ordinary working instrument, how- 

 ever, this improved Nachet Binocular can scarcely 

 be equal to that of Wenham or Stephenson; whilst 

 it must be regarded as inferior to the former in the 

 following particulars: First, that as the uninterrupted 

 half of the cone of rays (when the interposed prism 

 is adjusted for Stereoscopic vision) has to pass through 

 the two plane surfaces of the prism, a certain loss Na scopic 

 of light and deterioration of the picture are neces- 

 sarily involved; whilst, as the interrupted half of the cone of rays 

 has to pass through four surfaces, the picture formed by it is yet 

 more unfavorably affected; second, that as power of motion must be 

 given to loth prisms to A, for the reversal of the images, and to B for 

 the adjustment of the distance between the two bodies there is a greater 

 liability to derangement. 2 It does not give the equal illumination of Mr. 

 Stephenson's, is less free from optical error, and cannot, like his, be 

 used with high powers. 



39. The Stereoscopic Binocular is put to its most advantageouse use, 

 when applied either to opaque objects of whose solid forms we are 

 desirous of gaining an exact appreciation, or to transparent object* 

 which have such a thickness as to make the accurate distinction between' 

 their nearer and their more remote planes a matter of importance- 

 That its best and truest effects can only be obtained by objectives not 

 exceeding 40 of angular aperture, may be shown both theoretically and! 

 practically. Taking the average distance between the pupils of the two 

 eyes as the base of a triangle, and any point of an object placed at the 

 ordinary reading distance as its apex, the vertical angle inclosed between 

 its two sides will be from 12 to 15; which, in other words, is the angle 

 of divergence between the rays preceding from any point of an object at 



1 "Monthly MicroscopicalJournal," Vol. i. (1869), p. 31. 



2 M. Nachet's arrangement, like Mr. Wenham's, can be adapted to any existing 

 Microscope ; and it seems peculiarly suitable to those of French or German con- 

 struction, in which the body is much shorter than in the ordinary English models. 

 For in the application of the Wenham arrangement to a short Microscope, the 

 requisite distance between the eye-glasses of its two bodies can only be obtained 

 by making those bodies converge at an angle so wide as to produce great discom- 

 fort in the use of the instrument, from the necessity of maintaining an unusual 

 degree of convergence between the axes of the eyes. 



