242 W. LeConte Stevens — Physiological Optics : 



ing to the physical cabinet of Columbia College, the use of 

 which was kindly granted by Professor 0. N. Kood. In the 

 secondary circuit a Leyden jar was interposed, and by means 

 of a pair of lenses the light was separately converged upon each 

 of the pair of pictures, strongly il 



in the dark room. Each picture was kept upon the pivoted 

 arm of the stereoscope at a fixed distance from the oblique 

 mirror that reflected its light into the eye of the observer. 

 The sum of the incident and reflected rays from each to the 

 receiving eye was, as nearly as possible, 25 cm. 



The stereograph first employed was one of the moon, the 

 same one formerly used with this instrument in vision by con- 

 tinuous light. By varying the arrangement of the cards the 

 visual effect of binocular combination of images could be made 

 that of either a convex surface, or concave surface, or an indis- 

 tinct but flat surface. The observer placed himself with closed 

 eyes in proper position before the stereoscope while the manip- 

 ulator of the apparatus arranged the cards. He was then 

 requested to interpret the combined retinal image produced by 

 k, not knowing previ- 

 ously whether to expect convexity, concavity, or flatness in 

 the combined picture, and the interval of illumination being 

 too brief for any possible play of the eyes. 



In conducting these experiments I was so fortunate as to 

 secure the cooperation of Mr. W. W. Share, assistant Professor 

 of Physics in Columbia College, who soon acquired more than 

 usual "skill in the control of his eyes. Each of us acted suc- 

 cessively us observer and as manipulator of the apparatus, the 

 work being distributed through a number of days for the 

 purpose of avoiding fatigue at any one sitting. When the 

 relation between the visual lines was such as to imply no 

 unusual muscular strain, each of us found it possible to inter- 

 pret the binocular retinal image correctly by tbe light of a 

 single spark. Many other stereographs were substituted in 

 •: for that of the moon, and with similar results. 

 Some of these consisted of heavy black lines on a white ground, 

 others of white lines on a black ground ; in some cases one 

 picture belonged to one of these classes and its mate to the 

 other. At the suggestion of Professor Kood a pair were con- 

 structed, one >A' which consisted of green lines on a red ground, 

 the other of red lines on a green ground. In this case of com- 

 plementary colors it was a little more difficult to attain a per- 

 fectly clear perception b v a single spark, but when there was 

 any uncertainty another spark after an interval of a second of 

 time, was usually sufficient to resolve the doubt These pic- 

 tures were arranged to give stereoscopic relief, but the nature 

 of this, whether direct or inverse, was what the observer had 

 to determine. 



