34: THE MICROSCOPE AND ITS REVELATIONS. 



body r', passing out of the prism perpendicularly to the plane of emersion, 

 which has such an inclination that the right-hand or secondary body (K* 

 Fig. 28) may diverge from the left or principal body at a suitable angle! 

 On the other hand, the right half (r) of the cone of rays passes upwards, 

 without essential interruption, through the two parallel surfaces of the 

 prism A, into the left-hand body (/'), and is thus crossed by the other in 

 the interior of the prism. But if the prism A be pushed over towards the 

 right (by pressing the button a, Fig. 28), so as to leave the left half of 

 the objective uncovered (as shown in Fig. 27, 2), that half (l f ) of the 



PIG, 27. 



Arrangement of Prisms in Nachet's Stereo-pseudoscopic Binocular:!, for Stereoscopic; 2, fop 

 Pseudoscopic effect. 



cone of rays will go on without any interruption into the left-hand, body 

 (?'), whilst the right half (r r') will be reflected by the oblique face of the 

 prism into the horizontal direction, will emerge at its vertical face, and, 

 being received by the second prism B, will be directed by it into the 

 right-b&nd body (r'). Now, in the first position, the two halves of the 

 cone of rays being made to cross into the opposite bodies, true Stereo- 

 scopic relief is given to the image formed by their recombination, just as 

 in the arrangements previously described. But when, in the second 

 position, each half of the cone passes into the body of its own side, so 

 that the reveasal of the images produced by the Microscope itself ( 26) 

 is no longer corrected by the crossing of the two pencils separated by the 

 prism A, a Pseudoscopic effect, or ' conversion of relief,' is produced, the 

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

 its concavities being 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. 1 As originally constructed, the adjustment for dis- 



1 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 conclusion of 31. Where, as 

 in the case of the saucer-like disks of the Arachnoidiscus (Plate xii.), the real and 

 the converted forms are equally familiar, the ' conversion ' either of the convex 

 exterior or 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 becom- 

 ing complete. And there are some objects which resist conversion altogether, the 

 only effect being a confusion of the two images. 



