on Physiological Optics. 313 



which it was possible to attain such results without motion of 

 the eyes, it was important to employ the electric spark as a 

 means of illumination. The opportunity of doing so was then 

 wanting, but has since been secured. 



Vision by the Light of the Electric Spark. 



The apparatus employed for the production of momentary 

 illumination was a large induction-coil belonging to the Phy- 

 sical Laboratory of Columbia College (New York), and loaned 

 for the purpose by Professor 0. N. Rood. The stereoscope 

 used was the reflecting instrument described in a former 

 paper*, which had been so constructed as to give for registra- 

 tion the angle, positive or negative, between the observer's 

 visual lines, the distance of each card from the eye that receives 

 its image, and the angle which the plane of this card makes 

 with the visual line, assuming the latter to be horizontal and 

 the axis of rotation of the card to be vertical. The writer was 

 fortunate in securing the cooperation of Mr. W. W. Share, 

 Assistant in Physics in Columbia College, who soon acquired 

 more than usual skill in the control of his eyes for binocular 

 experiments. 



In the dark room the stereoscope was first so arranged that 

 parallelism between the two visual lines was necessitated, in 

 obtaining binocular vision of the pair of pictures at the moment 

 these were equally illuminated by the passing of a spark The 

 plane of each card being perpendicularly across the support- 

 ing arm of the reflector, the binocular resultant presented the 

 appearance of a series of concentric circles on a flat surface. 

 By rotating each card through a known angle on its vertical 

 axis, the binocular resultant could be made to assume at will 

 the form of a convex or concave elliptic shield. The observer 

 was seated with closed eyes in front of the stereoscope while 

 the manipulator of the apparatus arranged the cards. The 

 observer, not knowing whether this arrangement would pro- 

 duce planeness, convexity, or concavity, was then invited to 

 open his eyes and interpret the binocular retinal sensation 

 attained by the illumination of the cards with a single spark. 

 It was found possible, nearly always, to make a correct inter- 

 pretation at the first trial. Mr. Share and the writer acted 

 each -alternately as observer and manipulator; and the result 

 attained was confirmed by the experience of Professor Rood, 

 who tried the same experiments independently. 



The distance and diameter of the circle on each card being- 

 known, and also the angle of rotation on its vertical axis, it 

 becomes possible to calculate the maximum retinal displace- 



* Philosophical Magazine, December 1881. 



