286 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



produce a surprising effect in the stereoscope. The single image 

 generally gives the idea of a confused agglomeration of grey patches, 

 whereas the stereoscopic combination brings out in the most palpable 

 manner the forms of the blocks, as well as the effects of trans- 

 mitted and reflected light. The primary difficulty lies in accurately 

 rendering such irregular forms as blocks of ice when simply illu- 

 minated by incident light; this is increased by the light trans- 

 mitted by the ice, which completely alters the ordinary effects 

 of shadows. The stereoscopic representation of brilliant objects, 

 such as water covered with light rippling waves, produces very 

 wonderful effects." 



Some stereoscopes are so constructed that the light proceeding from 

 the two pictures, before passing through the prisms or eye-lenses, is 

 totally reflected inside two rectangular prisms, whose reflecting sur- 

 face is parallel to the direction of the light which reaches the eyes. 

 In this arrangement, the two images are seen with the symmetry of 

 nature ; they are superposed, but in such a way that what is on 



the right side is seen on the left, and 

 vice versa. The images are thus in- 

 verted ; and, consequently, the result 

 is such that hollow objects appear in 

 relief, and the reliefs appear hollow. 



Nevertheless the shadows sometimes 

 dispel this illusion, as do other circum- 

 stances which assist the perspective and 

 shadows in giving the vision the feeling 

 of relief. An example will show us 

 the reason for the change of position 

 of the images in this arrangement of 

 the stereoscope, which is called the 

 pseudoscope. 



Let us consider the case of a truncated pyramid seen from above, 

 and let us suppose that the oblique light produces no shadow ; 

 there will only be the various degrees of brightness in the lateral 

 surfaces. 



The two stereoscopic views should be arranged as in figures A' and 

 A", and then they will give, in the stereoscope, the impression of 

 relief. But in the pseudoscope the two drawings give symmetrical 



Fro. 214. The pseudoscope. 



