620 



SCIENCE. 



[N. S. Vol. VII. No. 175. 



sonal note adds the information that a thick 

 cylindrical lens was used about If by IJ 

 inches, with the concave surface a section 

 of a circle, 6 inches in radius ; that these 

 lenses were fixed about one inch away from 

 the stereoscopic lenses and about 3 inches 

 from the picture. 



I have prepared a pair of pseudo-stereo- 

 graphs from the one-half of a true stereo- 

 graph, and could thus directlj'^ compare the 

 life-likeness in the two cases. The advan- 

 tage is entirely on the side of the true 

 stereograph, not only on account of greater 

 technical precision in the photographic 

 plates, but because the degree and distribu- 

 tion of the dissimilarities of an actual view 

 photographed by a stereoscopic camera 

 more clearly imitate the retinal dissimilari- 

 ties than do the two views of a photograph 

 held at opposite angles with the camera. 

 When I compare the result in the pseudo- 

 stereographs with that of an arrangement 

 like that described by Sir David Salomons 

 (which, however, I have reproduced in 

 general, not in precise detail) I regard the 

 former as giving the better result. All 

 these processes, however, are limited in 

 scope and demonstrate that it is possible to 

 produce a stereoscopic eifect in a single 

 picture only in so far as such a picture may 

 be made to jield a pair of appropriately 

 dissimilar views. 



In this connection may be mentioned a 

 device published in the set of Psendoptics 

 of Professor Miinsterberg. It consists of a 

 card suitably shaped to be held against the 

 forehead and the ridge of the nose, so that 

 the diagrams printed on the two sides of 

 the card maj"- be seen at close range, the 

 one by the left eye, the other by the right, 

 and then combined by projection outward 

 upon a common imaginary plane. To allow 

 for the fore-shortening of lines at this close 

 range as compared with their projection, 

 the vertical lines of the diagram are exag- 

 gerated in thickness as compared with the 



horizontal lines, and the nearer lines are 

 proportionately heavier than the farther. 

 This device presents a stereoscope in its ut- 

 most simplicity ; but still it includes the 

 combination of a pair of appropriately dis- 

 similar views, and provides that each eye 

 shall see only its own view. In the same 

 connection may be mentioned another very 

 simple device which is very useful for 

 demonstration, but has not been generally 

 described.* It may also be found in the 

 set of Psendoptics and consists of a pair of 

 tubes about \\ inches in diameter and 8J 

 inches long, over the ends of which are 

 placed caps which contain on transparent 

 paper the pair of stereoscopic diagrams. 

 The tubes are simply held one before each 

 ej'e and are rotated until the two diagrams 

 are superimposed, when a stereoscopic com- 

 bination takes place by simple convergence. 

 This device is again limited in scope, for 

 the diagrams must be small and not too 

 elaborate. 



Some years ago, in verifying the possi- 

 bility, or rather the impossibility, of pro- 

 ducing a true stereoscopic effect with a 

 single picture, I attempted to utilize the 

 principle of the tel ester eoscope. This prin- 

 ciple consists in the reflection of images 

 first from a pair of mirrors which meet at 

 the point between the eyes and there form 

 an angle of about 80°, and then again 

 from a pair of mirrors farther away to the 

 right and left and parallel to the first set. 

 The ordinary stereograph is then viewed by 

 reflection from the mirrors. In this stereo- 

 scope the path of the rays is long and the 

 picture appears diminished in size. I have 

 recently constructed such an apparatus 

 with the two outward mirrors set on a 

 pivot, with which I can view either single 

 pictures or a pair of stereoscopic ones. 

 This apparatus is, therefore, both a stereo- 

 scope and a perspectoscope, if by the latter 



* It is described in the ' American Text-book of 

 Physiology ' (1897), p. 802. 



