132 Proceedings of the Royal Society 
covered my closed eyes with my hand, so as to cause complete 
darkness. If I now removed my hand, the eyelids still being closed 
the crimson flush made its appearance ; the darkness having been 
continued for a considerable time. I soon found that if the closed 
eyes were first directed to a white handkerchief, and then to the 
bright sky, the crimson flush made its appearance. At this stage 
the true explanation of the phenomenon began to appear. It was 
that the colour of white light that has passed through the eyelid 
is dingy orange or yellowish brown, and that the crimson flush is 
due to T ait’s property of the retina, namely, that when the eye is 
suddenly illuminated , or when the illumination is suddenly increased , 
the retina first acquires the qjoiver of recognising the deep red ; but the 
other colours usually follow so rapidly as to prevent this fact from 
being recognised. I hope that Professor Tait will allow me to make 
this slight addition to his statement, as originally made. 
According to this theory, the reason why this flush is only some- 
times seen is, that peculiarly favourable circumstances are neces- 
sary for observing it. These are (1), a very long rest to the eye, 
(as this is how Professor Tait and Professor Crum Brown saw it) ; 
or (2), a very sudden illumination of the retina (this is the experi- 
ment of the gas-flame described in the first part of this communi- 
cation) ; or (3), an exposure to a very feeble light after the eye 
has been in the dark for a short time (this is what I have just de- 
scribed). To prove still further that this, and not the transmission 
of light through blood, is the true explanation of the crimson flush 
as usually seen, I tried the following experiment : — A piece of 
common whitey-brown paper, four folds thick, was placed in front 
of one eye (the other being quite darkened). This shaded eye was 
kept dark for a short time, then keeping it closed to the skin to 
prevent stray light from entering, the head was raised, and the 
eye opened pointing to the sky. The crimson flush was un- 
precedentedly vivid, but soon yielded to the yellow colour of the 
paper employed. Lastly, six folds of plain white glazed writing 
paper were placed in front of the eye in the same manner. A 
longer duration of darkness was necessary than in the last case, 
but then the crimson flush was well shown, the colour then changed 
to orange, and it was some time before it assumed its natural white 
colour. 
