PROCEEDINGS OF SOCIETIES. 175 
risons, as by using both eyes alternately one eye could be kept 
comparatively fresh, and this alternation would be favorable to 
the prolonged examination of exceedingly delicate objects. He 
thought, too, that Mr. Wenham’s invention would be exceedingly 
valuable for objects which required the stereoscopic effect of com- 
bining two dissimilar images. He presumed it would not be 
doubted that this result would be obtdined. No one could look 
at the moon through an ordinary stereoscope and say it appeared 
flat; and he (Mr. Slack) had never met with any person who 
could see even an approximation to flatness, and though in his 
own case he could not say that the invention produced the stereo- 
scopic effect of the combination of two dissimilar objects, it 
certainly did not give the idea of flatness. Mr. Slack concluded 
by asking Mr. Wenham whether in his arrangement each eye got 
exactly the same proportion of marginal and peripheral ray, as a 
difference in this respect might produce a difference in result. 
Mr. Wenuam said that with the eight and twelve and higher 
powers the images would be identical. On drawing out the 
prism it would simply cut off a portion of the field of view, and 
this would be done simultaneously with both eye-pieces. 
Mr. Gray thought Mr. Slack had combined two things which 
were essentially different—the stereoscopic effect produced by 
single vision, such as looking through a telescope, and the effect 
of viewing the same object binocularly. There were two ways of 
arriving at a conclusion whether an object was flat or not. In the 
case of the moon they would see the shadows of that body. 
Mr. Wrenuam.—I spoke of the full moon. 
Mr. Gray continued—If any one doubted that there was a 
difference between real stereoscopical vision and inferential stereo- 
scopical vision, let him examine stereoscopic slides of the moon 
where a corresponding image was taken of two extremities. He 
could not say that the moon would appear flat through a telescope ; 
it would appear as if reduced into a small globe; but in the 
stereoscopic slides they saw more than a hemisphere, say three 
quarters of the diameter, and bearing this difference in mind would 
simplify the distinction between real stereoscopical and inferen- 
tially stereoscopical vision. Mr. Slack had mentioned that one 
of his eyes differed from the other in focus, and he therefore pro- 
bably did not fully appreciate the effect that most persons received 
from a stereoscopic picture, and was therefore the less able to 
distinguish between the inferentially stereoscopic and real stereo- 
scopic vision. 
Mr. Becx said that Mr. Slack’s remarks confirmed his opinion 
that the moon could not be seen as flat through an ordinary tele- 
scope, but he thought it would be seen flat through a binocular 
telescope. The effect of the binocular telescope, with its two 
telescopes placed any farther apart than the ordinary distance of 
the eyes, would be rather to diminish stereoscopic vision. For 
instance, a tolerably near object, such as a flower-pot standing 
outside a window, would, with an ordinary-vision telescope, appear 
