507 
of Edinburgh, Session 1874 - 75 . 
colours of purple or green ; on the contrary, as was stated in the 
paper alluded to, at one time the coloured slips appear alternately 
visible, at another time one half of each may be visible, and occa- 
sionally, spots, or it may be only minute specks, smaller even than 
the fifth part the diameter of a small pin head of the one colour, 
will be seen shining on the ground colour of the other card-board. 
These particular changes seem to depend greatly on the excited or 
the fatigued condition of the retinae at the time; for if we direct 
our attention to any conventional mark made on either of the slips 
presented, the. excitement of the retina of the eye, caused by the 
act of observing the mark, immediately causes the slip on which the 
mark is made to become visible, and the mark appears surrounded 
with a patch of the colour of the slip on which it is placed. 
Two circumstances then may be mentioned as certain : that in 
no instance do the two colours blend into an intermediate colour ; 
and second, that we never observe the same portion of the bright over- 
lapping portion to have at one and the same moment two different 
colours; parts or spots or minute specks may, as we have said, 
appear of the one colour, and other parts may appear of the other 
colour, but though the one coloured slip visually overlaps the other 
differently coloured slip, we never see any part at once to possess 
two different colours. 
The conclusions to which these phenomena lead are certainly 
these — that there is a physical union in a cerebral lobe of the 
nerve impressions coming from the two eyes, and in no other way 
can we account for the two retinal images giving the mind the im- 
pression of but one object both in natural and stereoscopic vision 
when corresponding retinal fibres are excited, and of double objects 
when non-corresponding fibres are excited — and no other suppo- 
sition will account for the increased brightness obtained by the use 
of two eyes than that suggested, namely, that the nerve impressions 
from both eyes are physically united in the sensorium. 
Another conclusion to which we are led is, that though the cor- 
responding retinal fibres are brought into juxtaposition in the 
sensory, yet they are not there joined or amalgamated the one 
with the other, seeing they do not produce the effect of an inter- 
mediate colour, but each fibre transmits to the sensory the distinc- 
tive colour and impression which it receives in the retina. 
