PHYSICS OF THE NINETEENTH CENT.-OPTICS. 517 



It is formed of two, or better, three plane mirrors inclined to each 

 other at angles of 60. However irregular may be the form of the 

 small objects placed in one end of the tube of the kaleidoscope, the 

 reflections convert them into beautiful sym- 

 metrical patterns. An example is given in 

 Fig- 235, in which the central equilateral tri- 

 angle contains the real objects. The kaleido- 

 scope was invented in 1816 by Sir DAVID 

 BREWSTER (1781 1853), to whom opticians 

 are indebted for many other ingenious inven- 

 tions and interesting discoveries. 



The stereoscope in its ordinary and well- 

 known form (Fig. 236) is also an invention of 

 Sir D. Brewster's. In its original form, in- 

 vented by Professor Wheatstone (Fig. 236), 

 the pictures were viewed by reflection. The 

 impression of relief in the objects represented 

 in stereoscopic pictures is obtained by the 

 circumstance of each eye seeing a slightly dif- 

 ferent picture, in which the differences exactly 

 reproduce those of the images to which the 

 real objects would give rise. 



Since 1872 Mr. Crookes, the discoverer of thallium (page 488), has 

 been engaged in a long series of investigations, which have already 

 revealed to us the existence of peculiar and hitherto unknown con- 



FIG. 236. BREWSTER'S 

 LENTICULAR STEREO- 

 SCOPE. 



FIG. 237. WHEATSTONE'S REFLECTING STEREOSCOPE. 



ditions of matter. The opening out of a new field of research is 

 always an interesting circumstance in the history of science, and some 

 of the facts which Mr. Crookes has arrived at are so extraordinary 

 that a boundless prospect of the future insight into the most recondite 

 and subtle operations of nature, which may yet be obtained, rises 

 before the imagination. The facts Mr. Crookes has discovered have 

 for the most part been beyond the power of our received theories of 

 light, heat, or " molecular actions " to predict, and the theoretical ex- 



