356 Proceedings of the Royal Society 
To prove that there are two mental impressions, let us re- 
verse one of the coins. When this is done, we have no longer 
the impression of one coin, but of two coins occupying the same 
place. Both are visible, and they appear as if the one were visible 
through the other. While we steadily regard this anomalous 
presentation, the eye and the judgment become alike puzzled by 
it, and an effort is made to reduce the phenomenon to a normal and 
intelligible object of vision ; a succession of transformations is the 
result of the joint action of the mind, and of the disturbed nervous 
centres which ensues; at one moment we see one coin, and then, 
suddenly, it disappears, and the other takes its place ; then we see 
both coins at once, or a part of each perhaps becomes alone visible. 
In ordinary vision, then, we must conclude that objects make an 
equal impression on the identical points of each retina, though we are 
not intellectually conscious of the fact of duality ; and the question 
thus arises, If there are two retinal impressions, how do we account 
for the two appearing as if superimposed the one on the top of the 
other? The eyes are set apart in the head, and the supposed 
sensory ganglia at the base of the brain, the corpora geniculata, the 
corpora quadrigemina , and the optic thalami, are all in duplicate : 
and the cerebral hemispheres divide the head in two equal sections. 
How, then, are we to account for the two visual images being 
united? It has been very generally assumed that the mind com- 
bines the two impressions, as it were, into one. This is the 
opinion of Professor Wheatstone and Dr Carpenter, and it was for 
many years my opinion ; but the phenomena about to be alluded 
to convinced me that I was wrong, and that there exists a physical 
cause for the union of the two images ; and to prove this is the 
main purpose of the paper. 
When we take two strips of white card-board about an inch 
broad, and insert one at each side of the stereoscope, we find that 
each strip is distinctly seen by each eye ; but when we cause them 
gradually to approach till the two ends appear to overlap say an 
inch or more, the effect is singular. Where the strips seem over- 
lapping, the brightness is observed instantly to become very much 
increased : so much so, indeed, that when we fix the attention on 
the quadrangular part formed by the overlapping ends, all the rest 
of the strips become invisible, and the overlapping parts alone 
