May 6, 1898.] 



SCIENCE. 



617 



is one incli in height and the other two 

 inches, we infer (under proper conditions 

 of perspective) not that the one man is 

 twice as tall as the other, but that the two 

 are of approximately similar size, and that 

 the one is considerably more distant. As 

 a further factor one may mention the in- 

 terrupted view of a more distant object by 

 reason of a nearer object standing in front 

 of it. Most objects are opaque, and on this 

 account we infer the continuity of outlines 

 which are more or less hidden by the ob- 

 jects in the foreground. If, for example, we 

 see in reality or on a picture a bush in front 

 of a fence we do not infer that the fence is 

 broken where the bush prevents us from 

 seeing it, but that it is continuous and far- 

 ther away than the bush. 



When all these factors cooperate they 

 produce a very complete illusion of depth, 

 and frequently one which does not seem to 

 require the operation of the more physio- 

 logical factors of convergence and the dif- 

 ference in the retinal images. In the case 

 of photographs viewed through the stereo- 

 scope we have the combination of all the 

 above factors, and it requires a rather de- 

 tailed analysis to make clear the influence 

 of each. It is possible, however, to prove 

 conclusively that the difference in the ret- 

 inal images is the prime factor and that all 

 the others form accessory methods for the 

 inference of depth, but are not at all neces- 

 sary for this effect. For this purpose we 

 must have stereoscopic views which show 

 no light and shade, no perspective, no in- 

 terposition of objects. Geometrical figures 

 theoretically constructed have been gener- 

 ailly used for this purpose. A very superior 

 ■series of diagrams has recently been pub- 

 lished and forms an important aid to the 

 study of stereoscopic vision. They are 

 the result of the application of the ste- 

 reoscope to the demonstration of math- 

 ematical problems, a result which has been 

 most ingeniously reached by Professor 



C. S. Slichter,* of the University of Wis- 

 consin. These diagrams represent the mo- 

 tion of a point in space under the influ- 

 ence of three forces acting respectively in the 

 three dimensions of space. A small electric 

 lamp suspended in a dark room is given a 

 pendular motion, and at the same time a 

 stereoscopic camera is itself swung in a di- 

 rection at right angles to the motion of the 

 lamp. These movements are brought into 

 unison by means of electro-magnets, and 

 the result is that the point of light leaves 

 its trace on the pair of photographic plates 

 precisely as though a pair of eyes were fol- 

 lowing the movement of the point in and 

 out through the three dimensions of space, 

 but that in addition the track of this point 

 of light is retained from beginning to end. 

 These views thus represent beautiful and in- 

 tricate mathematical curves and in the stere- 

 oscope appear distinctly three-dimensional 

 as wire forms or models. I know of noth- 

 ing which equals these views in clearness 

 and precision, and I cordially recommend 

 them as test diagrams in stereoscopic ex- 

 periments. It may be said that any instru- 

 ment which succeeds in producing from a 

 pair of such views a full and complete ap- 

 pearance of depth is a true stereoscope, and 

 one which fails to do this is not a true and 

 perfect stereoscope. 



An interesting and novel type of appa- 

 ratus has recently been introduced under 

 the tei-m of Perspectoscope. It consists in. 



The Peespectoscope. 



* Transactions of the Wisconsin Academy, 1898, 

 Vol. XI., p. 449. 



