618 



SCIENCE. 



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



the main of two eye- pieces fitted with 

 suitable lenses and of a pair of mirrors, 

 the two mirrors being set at such an 

 angle that the image from a single pic- 

 ture which is placed at right angles to 

 the eye pieces will be reflected into each of 

 the eyes. The accompanying illustrations 

 wUl readily make clear the principle. The 



Peinciplk of the Peespectoscope. 



B. B.' — Viewing tubes. C. C — Lenses. D. D.'— 



Eeflectors. E. E. — Picture Holder. F. — Picture. 



inventor of this instrument claims that it 

 disproves the accepted theory of the stere- 

 oscope, because with it one can see a single 

 picture, such as any ordinary photograph 

 or drawing, in apparent perspective. He 

 further claims that in this way a true per- 

 spective is obtained, while the ordinary 

 stereoscope is alleged to exaggerate the per- 

 spective.* These claims can be readily dis- 

 proved ; in the first place, the perspectoscope 

 utterly fails to exhibit the test diagrams 

 above described as solid ; secondly, the or- 

 dinary stereoscope does not as a rule ex- 

 aggerate the perspective, although such an 

 exaggeration may be readily obtained if 

 desired. In Professor Slichter's diagrams 

 the motion from left to right was in reality 



* He also argues that the perspectoscope obviates 

 the necessity of extreme convergence as the eyes as- 

 sume a natural position. This is true, but the objec- 

 tion holds against the Brevfster stereoscope, not 

 against all others. Further, this advantage is here 

 gained at the expense of inverting the picture from 

 right to left. He makes other inadmissible state- 

 ments, which it is not necessary to consider here. 



equal in extent to the motion forward and 

 backward. I asked a number of persons as 

 they viewed these diagrams to estimate the 

 breadth in terms of the depth, and the gen- 

 eral tendency was to regard the diagram as 

 somewhat broader than deep. Although 

 the claims made for the perspectoscope can- 

 not be allowed, it is true that when a photo- 

 graph is viewed by the average observer in 

 the perspectoscope there is a striking ap- 

 pearance of depth, quite enough to make 

 this apparatus a popular instrument for 

 viewing pictures. How is this eflect of ap- 

 parent depth produced? The answer is 

 in the main that the accessory factors in 

 the perception of depth are here introduced 

 at their maximum efficiency ; and added to 

 this there is the action of the lenses in 

 magnifying the objects, and the convenience 

 and precision with which the views as re- 

 flected from the two mirrors may be super- 

 imposed. It is a well-known fact that a 

 large magnifying glass is itself an important 

 aid in the perception of the third dimension 

 in a photograph, and this aid is utilized in 

 the perspectoscope in a more convenient 

 form. For the same reason the glasses in 

 the ordinary stereoscope are not prisms 

 but prismatic lenses. 



To one who is familiar with the appear- 

 ance in the stereoscope of a pair of stereo- 

 scopic views (or stereographs, as Le Conte 

 Stevens suggests) there is something decid- 

 edly lacking in the perspectoscope eflect; 

 and yet, unless the two are viewed imme- 

 diately in succession, the average observer 

 might well be misled to regard the perspec- 

 toscope as really producing the appearance 

 of depth. This is altogether likely to be 

 the case if the observer happens to use 

 stereographs which are not well made. 

 Unfortunately, a very large proportion of 

 the views commonly sold are far from per- 

 fect, and a considerable part of a dealer's 

 entire stock which I recently examined was 

 made up of views which were not stereo- 



