1909-10.] Prof. F. G. Baily on a Stereoscopic Illusion. 
551 
XL. — A Stereoscopic Illusion. By Professor Francis Gibson 
Baily, M.A. 
(Read February 7, 1910. MS. received March 14, 1910.) 
If the finger be held vertically in front of the eyes and a distant object be 
looked at, it is well known that two images of the finger will be seen, quite 
transparent if the two images do not overlap, and opaque only at overlap- 
ping parts. In place of the finger, use a thin rod at a distance of some 
6 or more feet, and focus on another vertical rod at a distance of 30 feet 
or more, so that the second rod is seen between the two images of the 
first. The apparent position of the second rod will be found to be distinctly 
nearer than its real position. 
In fig. 1, Aj A 2 are the observer’s eyes, B is the first rod and C the 
second. Each eye sees C without hindrance, and the observer sees a trans- 
parent image of B on each side. C then appears to be at G. 
To observe this, B is preferably not strongly lighted, or the tendency 
to focus on B may be too great. C must be well away from its background, 
so as to stand alone with the background out of focus. The ground on 
which C stands must be hidden, so as to prevent any other knowledge of 
its position. For if the actual situation on the ground is seen, the visible 
proof of its position will mentally outweigh the indication given by the 
stereoscopic effect. 
The apparent position of C may be determined by setting up a similar 
rod D near to the line A C, sufficiently on one side to be visible to both 
eyes. D is then moved backwards and forwards, until it is judged to be 
the same distance from the observer as C appears to be. The diagram 
has been drawn to correspond with the following actual figures, the 
vertical scale being exaggerated to twenty times the horizontal : — A 1 A 2 , 
2f inches ; B, 1 inch broad ; A C, 100 feet ; and C D, 1 foot. 
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