May 6, 1898.] 



SCIENCE. 



619 



scopic at all, the two impressions of the 

 photograph being iwedsehj alike. It would, 

 of course, be true that there would be no 

 difference between a pair of such pictures 

 viewed in the stereoscope and a single one 

 of them viewed in the perspectoscope. The 

 difference in this effect can readily be pro- 

 duced by any one who can procure two 

 copies of the same pair of true stereoscopic 

 pictures. Cut these in half and set up 

 first either the two right or the two left 

 halves, and then contrast this appearance 

 with that obtained by viewing a right and 

 left half. In this way the observer will 

 soon train himself to recognize the differ- 

 ence between a genuine stereoscopic effect 

 and one that only approximates it to a 

 greater or a less extent. In the one case 

 the object stands out with all the reality of 

 life, while in the other case there is a rel- 

 ative flatness and only a pictorial type of 

 perspective. It is something like the dif- 

 ference between viewing a model or a 

 tableau and a picture ; in the one case we 

 have the difference in the two retinal im- 

 ages, together, of course, with all the acces- 

 sory aids to the perception of depth, while 

 in the latter case we have all the accesso- 

 ries but not the main factor. This ex- 

 periment thus serves as an experimentum 

 cruds and further indicates that it requires 

 some little experience with stereoscopic 

 effects to enable one to judge between the 

 true appearance and those which more or 

 less successfully imitate them. 



It is quite an easy matter, however, to 

 make a true stereoscope out of the perspecto- 

 scope ; one need only make the reflecting 

 mirrors adjustable and set them so that the 

 one will reflect into the one eye one-half of 

 an ordinary stereograph and the other mir- 

 ror will reflect into the other eye the other 

 half. Or the same result may be produced 

 by a pair of fixed mirrors set at a suitable 

 angle to so direct the reflected images for 

 the stereograph of ordinary size. This 



form of construction for a simple and effec- 

 tive stereoscope has not, to my knowledge, 

 been described. 



The attempt to obtain a stereoscopic effect 

 from a single picture has been frequently 

 made, but in so far as it is successful it de- 

 pends upon securing two dissimilar views of 

 some picture which shall more or less closely 

 imitate the differences between the two 

 views of a stereograph. Le Conte Stevens* 

 has clearly indicated that by the combina- 

 tion of a pair of perfectly similar conjugate 

 pictures held inclined, like the two pages of 

 a partly opened book, one may obtain a 

 stereoscopic effect. In the same way photo- 

 graphs may be prepared from a single pic- 

 ture in which the picture is placed at an 

 angle with the plane of the plate ; and by 

 suitable shifting of the angle one may 

 secure two photographs of the original sin- 

 gle photograph which will present differ- 

 ences similar to those in the two halves of 

 a stereograph. This difference may be de- 

 scribed by saying that in the right-hand 

 view the left portion is somewhat crowded 

 together and the right portion somewhat 

 expanded, while the reverse is true of the 

 left-hand picture. A pair of views, thus 

 prepared, when placed in a stereoscope, 

 give an approximate stereoscopic effect. In 

 Nature (Feb. 3, 1898) Sir David Salomons 

 describes an arrangement of lenses which 

 will bring about such a distortion and will 

 thus produce from a single picture the 

 effects of depth. The device consists of 

 a pair of wedge-shaped piano-cylindrical 

 lenses, which, with their thicker edges set 

 together, are fixed in position near the two 

 prismatic lenses of an ordinary stereoscope 

 and between them and the picture. A per- 



* In the Philosophical Magazine, May, 1882. In the 

 same place is described a reversible stereoscope which 

 la much better suited to experimental purposes than 

 the ordinary stereoscope, and merits a more general 

 introduction in psychological laboratories than it has 

 as yet secured. 



