322 Mr. W. Le Conte Stevens on an 



of strict mathematical interpretation. His theory of visual 

 triangulation is inaccurate at best, and almost totally inappli- 

 cable in the very department to which it has been most applied 

 — that of stereoscopic perspective. 



40, W. 40th St., New York, 

 February 24, 1882. 



XXXVII. An Adjustable and Reversible Stereoscope. 



By W. Le Conte Stevens*. 



[Plate VIII. figs. 6 & 7.] 



THE experiments which formed the basis of my recent 

 papers on Physiological Opticsf were, for the most part, 

 made without the aid of any stereoscope, except the modifica- 

 tion of Wheatstone's reflector, with which various values of the 

 optic angle were measured. As few persons, however, have 

 sufficient muscular control over the eyes to test my results, I 

 devised an instrument which renders possible for untrained 

 eyes quite a number of experiments which with the ordinary 

 lenticular stereoscope would be either difficult or impossible. 

 The principal objects to be attained were as follows: — 



I. To secure ready motion to the semi-lenses, so that they 

 maybe adapted in position for any pair of eyes, whatever 

 may be the distance between the pupils, and for any stereo- 

 graph, whatever the interval, Avithin the usual limits, between 

 corresponding points on the two pictures. 



II. To secure the possibility of removing the semi-lenses at 

 will, so that they may be reversed in relative position, or 

 be replaced by prisms with their bases turned toward each 

 other, so that, without uncomfortable muscular strain, the left 

 picture may be viewed by the right eye and the right picture 

 by the left eye simultaneously, thus producing reversion of 

 relief in the binocular imnge, if desired. 



III. To secure ready motion to the screen, so that to each 

 eye either the whole stereograph, or its right half or its left 

 half separately, may be visible. 



IV. To secure the means of examining the binocular image 

 either alone or attended by monocular images, so that the dif- 

 ference between the two kinds of vision may be noted. 



V. To secure the means of using the same instrument either 

 with glasses or for binocular combination of images by direct 



* Communicated by tbe Author. 



t Proceedings of the American Association for the Advancement of 

 Science, Cincinnati Meeting, August 1881 ; Transactions of the New York 

 Academy of Sciences, Oct. 24, 1881, and Feb. 12, 1882 ; American Journal 

 of Science, November and December 1881 ; Philosophical Magazine, 

 December 1881. 



