May 6, 1898.] 



SGIENGE. 



621 



term we mean an apparatus which in one 

 way or another imitates a true perspective. 

 As it lacks lenses (although these could be 

 supplied), it does not yield such striking re- 

 sults as other stereoscopes, but is useful in 

 illustrating clearly the difference between a 

 combination of a pair of different views, for 

 the change can be made from one to the 

 other very quickly and without involving 

 any other modification. I am not aware 

 that the utilization of a telescope for this 

 purpose has been previously described.* 



In Hermann's ' Handbuch der Physio- 

 logie ' (3, p. 587) may be found a descrip- 

 tion of a device, originating with Hirsch- 

 berger, by which the picture is viewed near 

 by with the eyes in nearly a parallel posi- 

 tion, and thus an approximate stereo effect 

 is produced with a single picture. This was 

 accomplished by means of a pair of prisms ; 

 but as the arrangement is practically an in- 

 verted telestereoscope (with the outermost 

 pair of mirrors adjustable and spread to 

 the distance of the space between the eyes) , 

 I have used such an inverted telestereo- 

 scope for this purpose and with very good 

 success. The effect of depth is much bet- 

 ter than in the perspectoscope, especially 

 when a pair of lenses is used with this de- 

 vice, but it is not as effective as in a double- 

 picture stereoscope. Geometric diagrams 

 seem distinctly projected in space, and pho- 

 tographic representations are almost as 

 clear as in an ordinary stereoscope. In 

 other words, there are all degrees of the 



*I have amongst my stereoscopes one in which the 

 lenses are prismatic in one-half only, the other half 

 heing portions of true douhle convex spheres. By 

 the rotation of each lens to a definite position we can 

 use the apparatus simply as a pair of lenses and thus 

 view a single picture at the proper focus. The in- 

 strument is called a stereo-graphoscope. In the latter 

 form it is intended to accomplish just what the per- 

 spectoscope accomplishes, hut it is not so convenient. 

 The apparatus is very convenient as a stereoscope, be- 

 cause it admits of some adjustment of the positions 

 of the two halves of the stereograph. 



stereoscopic illusion from flatness to perfect 

 solidity, and this device represents about 

 the maximum degree obtainable with a 

 single picture. 



Another rather recent contribution is the 

 combination of the stereoscope with the 

 kinetoscope, thus producing the illusion of 

 figures moving in three dimensions of space. 

 Professor Miinsterberg described such a 

 device* in which the effect was obtained by 

 viewing through a pair of series of slits of a 

 large disc a single series of stroboscopic 

 figures ; the pair of series of slits is so ar- 

 ranged that one eye looks through the one, 

 and the other eye through the other, and as 

 the slitted disc and also the one with the 

 figures rotate rapidly the two eyes obtain 

 slightly different views of the stroboscopic 

 figures ; but the images follow one another 

 so rapidly as to fuse and produce the illu- 

 sion of motion and of depth. In a recent 

 letter Professor Miinsterberg informs me 

 that the same effect may be produced by the 

 use of a disc with one slit for both eyes and 

 a mirror held a few inches behind the disc; 

 for every slit there correspond two pic- 

 tures drawn on the back of the disc, which 

 when seen in the mirror furnish the appro- 

 priate pair of stereoscopic views. He also 

 suggests that the same may be done by 

 spinning such a disc upon a mirror with 

 appropriate illumination. Dr. Sanford has 

 also constructed a device for obtaining a 

 stereo-stroboscopic effect. 



The problem of projection by the lantern 

 of stereoscopic pictures is receiving renewed 

 consideration. The two methods most in 

 vogue are those of the double lantern with 

 the one view seen through green and the 

 other through red light, and the other by 

 application of polarized light. It is prob- 



*See Psyelwlogkal Review, 1894, I., p. 56. Also 

 Scripture, ' The New Psychology, ' pp. 431-435, where 

 this and similar devices by Sanford {American Journal 

 of Psycliology, 1894, p. 576) and Dvorak are also 

 described. 



