nachet's stereo-pseudoscopic binocular. 37 



But when, in the second position, each half of the cone passes 

 into the body of its own side, so that the reversal of the images 

 produced by the Microscope itself (§ 20) is no longer corrected by 

 the crossing of the two pencils separated by the Prism a, a Pseu- 

 doscopic effect, or 'conversion of relief,' is produced, the projec- 

 tions of the surface of the object being represented as hollows, and 

 its concavities turned into convexities. The suddenness with which 

 this conversion is brought about, without any alteration in the 

 position either of the Object or of the Observer, is a phenomenon 

 which no intelligent person can witness without interest ; whilst it 

 has a very special value for those who study the Physiology and 

 Psychology of Binocular vision.* As an ordinary working instru- 

 ment, however, this improved Nachet Binocular can scarcely be 

 said to possess any point of superiority to the Wenham ; whilst 

 it must be regarded as inferior in the following particulars : — First, 

 that as the uninterrupted half of the cone of rays (when the in- 

 terposed prism is adjusted for Stereoscopic vision) has to pass 

 through the two plane surfaces of the prism, a certain loss of light 

 and deterioration of the picture are necessarily 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 unfavourably 

 affected ; second, that as power of motion must be given to both 

 prisms — to a, for the reversal of the images, and to b for the ad- 

 justment of the distance between the two bodies — there is a greater 

 liability to derangement than in the simpler construction of Mr. 

 Wenham. f 



30. The Stereoscopic Binocular is put to its most advantageous use, 

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

 desirous of gaining an exact appreciation, or to transparent objects 

 which have such a thickness as to make the accurate distinction 

 between their nearer and their more remote planes a matter of im- 

 portance. That its best and truest effects can only be obtained by 



* The result of the numerous applications which the Author has made 

 of this instrument to a great variety of Microscopic objects, has led to a 

 confirmation of the principle of Pseudoscopic vision, stated at the con- 

 clusion of § 24.— Where, as in the case of the saucer-like disks of the 

 Arachnoidiscus (Plate x.), the real and the converted forms are equally 

 familiar, the ' conversion ' either of the convex exterior or of the concave 

 interior is made both suddenly and completely. In more complex and less 

 familiar forms, on the other hand, the conversion frequently requires 

 time ; being often partial in the first instance, and only gradually be- 

 coming complete. And there are some objects which resist conversion 

 altogether, the only effect being a confusion of the two images. 



t This arrangement, like Mr. Wenham's, can be adapted to any existing 

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

 German construction, 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 diverge at an angle so wide as to produce great discomfort in the 

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

 degree of convergence between the axes of the Eyes. 



