554 
Proceedings of the Eoyal Society of Edinburgh. [Sess. 
results were consistently low, but very irregular and uncertain in deter- 
mination, repeat readings not agreeing well. 
The image C' appears to come up to its final position somewhat slowly, 
the best results being obtained when the gaze is focussed on the space 
between C 7 and D, so as to have both equally in vision. Moving the eyes 
from one to the other did not give such conclusive impressions. 
The author has asked several people to observe the phenomenon, and 
they have found little difficulty in qualitative observation, though some 
have found trouble in keeping the focus on the distant object. Quantitative 
work seems more difficult, and will doubtless depend on the stereoscopic 
perception of the observer. The author had previously been tested for this 
power, and was found to have a perception above the average ; some people, 
on the other hand, are markedly deficient, and any defect of sight, such as 
would cause a person to attend to the impressions of one eye more than the 
other, would probably prevent the phenomenon being seen. Some power of 
control over the direction and focus of the gaze is essential, and this is not 
possessed equally by all people. 
It is obvious that the effect is physiological or psychological, and is not 
due to diffraction at the edge of the nearer rod, or any phenomenon 
external to the eye. Its simple proportionality above 20 feet distance and 
its complete cessation below 20 feet are remarkable ; but these figures refer 
only to the author’s eyesight, and may be entirely personal. He does not 
suggest any theory in explanation, but brings it forward in case it should 
be of interest to physiologists and psychologists. 
The phenomenon is probably another instance of the various optical 
illusions, more or less of a stereoscopic nature, which are well known and 
are described in textbooks on physiological optics. But its simplicity and 
adaptability to numerical measurement, its freedom from pictorial deception, 
and the large scale of the dimensions involved may render it suggestive in 
the theory of stereoscopy. This particular cause of illusion seems not to 
have been noticed before, even qualitatively. 
(. Issued separately October 12 , 1910 .) 
