of Edinburgh , Session 1 872 - 7 3 . 131 
be in the dark for more than an hour or so. But when the time 
of darkness is short, the crimson flush is seen only for a small 
fraction of a second. It is not necessary to have just awoken from 
sleep, though certainly this seems to favour the appearance, 
making it more extended and more lasting. The colour of this 
appearance is the same as that crimson flush which is often seen 
when the eyelids are closed and a light is shining on them. This 
struck both Professor Tait and myself, and led him to test whether 
it was due to the same cause, viz., the passage of light through 
the blood-vessels. 
****** 
I had proceeded thus far, and had moreover duly apologised to 
the Society for offering them a communication with so little 
novelty in it, when accident, or rather an inexcusable drowsiness, 
led me to perform some experiments that I look upon as of far 
greater importance, in that they give an extension to the property 
of the retina observed by Tait, in a direction quite unlooked for. 
When travelling in the train from Edinburgh to London lately, 
I had my eyes closed, and frequently saw that crimson flush which 
is so often seen under such circumstances, and to which I have 
already alluded. This has been stated by Professor Tait and my- 
self to be of the same hue as that observed by him in the cases 
mentioned in his note. It has always been attributed without 
any doubt to the passage of the light through the blood-vessels of 
the eyelid. But I soon noticed a remarkable fact, viz., that if the 
light of the sky remained of the same brightness, in other words, 
if the sun were not flitting behind clouds, this crimson flush gave 
place to a dingy orange or even yellowish brown colour. The 
brilliant crimson flush was in these circumstances seldom visible 
on closing the eyelids, and it invariably gave way to this dingy 
colour. On continuing to repeat this experiment, no doubt re- 
mained on my mind of the fact. Being now convinced that the 
appearance of white light passing through the blood-vessels of 
the eyelid is of this orange colour, I was at a loss to account for 
the crimson flush that is so often seen. I soon noticed, however, 
that when the eyes were closed, this brilliant colour never made its 
appearance, except at such moments as when the sun burst out 
from behind a cloud, thus brightening the field of view. I then 
