404 W. B, Rogers on Retinal Impressions. 
evaporated in a hemispherical porcelain basin, so as to form a 
net work of extremely slender needles, and these be viewed by 
gas light, the play of colors is singularly brilliant. 
2d. Dichroism. en by spontaneous evaporation of large 
quantities of solution of potash, or better, of ammonia salt, trans- 
parent prisms of j; to 5 inch diameter are obtained, these, 
viewed with a doubly refracting prism by transmitted light give 
abt images, one pale straw color, and the other deep brownish 
red. 
3d. The above described property of catachroism, or reflection 
in the plane of incidence of oppositely polarized beams. 
Philadelphia, August, 1860, 
/ Art. XL.—On our inability from the Retinal impression alone 
to determine which Retina is impressed ; by Prof. W1ni1aM B. 
ROGERS. 
[Read before the Am. Assoc, for the Advancement of Sci., at Newport, Aug. 1860.] 
ALTHOUGH on first view it might be supposed that an impres- 
sion made in either eye must necessarily be accompanied by a 
mental reference to the particular organ impressed, it will be seen 
from the following simple experiments that the impression of it- 
self is not essentially suggestive of the special retinal surface on 
which it is received. 
the right eye but not fairly in front of it, it will appear as if 
placed before the left eye, and by an additional motion bringing 
it fairly in front of the right eye it will seem to be equally be- 
fore both eyes or to be in the medial line between them. 
like effects may be observed by using a half sheet of rather 
stiff foolscap with a large pin hole in the centre. Bending this 
over the face and moving it until the hole is in front of one o 
