553 
1909-10.] Prof. F. G. Baily on a Stereoscopic Illusion. 
Fig. 2 shows three sets o£ readings under different conditions. Line 1, 
shown with dots, was taken on a bright day with C D kept small. Line 2, 
marked with crosses, was taken at a later date with a different back- 
ground (leaves off the trees) on a dull day, but with C D kept small as 
before. Line 3, with circles, was taken on the same day as line 2, but 
C D was made large. The lowest point of all, at 20 feet, was taken in the 
house by artificial light, carefully arranged so as to obtain very accurate 
results. 
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Fig. 2. 
The relation over this range of distance is clearly a close approximation 
to a linear function starting from a distance of about 20 feet. Before 
making quantitative measurements the author had tried to observe the 
phenomenon in a room, but without success. No movement of C could be 
detected. The subsequent quantitative determination showed that below a 
distance of 20 feet no effect would be visible. 
It will be noted that the values in line 3 are distinctly lowered by the 
increased distance between C' and D. A large number of readings were 
taken with irregular values of C D before its effect was identified, and the 
