219 Dr. G. J. Burch. [ Mar. 7, 
come up again and again in the most unexpected manner, after remaining 
invisible for half an hour or more. On one occasion, on my way to the 
laboratory, I had for some minutes watched a bird nest-building in a tree. 
After turning out the gas in the dark room I saw after-images of the flame 
for about 10 minutes. These died out, and for some time the room seemed 
full of a luminous fog. Then gradually an image of the branches round the 
birds’ nest developed, and persisted with more or less distinctness for nearly 
three-quarters of an hour. Under such circumstances, it is seldom worth 
while continuing the experiment, as the concluding stage is likely to be 
correspondingly delayed. It is better to avoid looking at the sky or any 
bright light for some hours before entering the dark room. 
_ If this precaution is taken, the average course of events is fairly shown by 
the following figures taken from seven experiments, in which the final stage 
was reached in two hours. The times given are the earliest and latest 
at which the phenomena described commenced in any of the seven. 
experiments :— 
0 to 5 minutes. Confused after-images of portions of objects recently looked at. 
These images, with the exception, perhaps, of that of the gas or lamp flame, do not 
usually appear for the first 20 or 30 seconds after the light is turned out. They 
have hard outlines, and they gradually fade and give place to a luminous fog, 
made up of what I have called ‘“ dazzle-tints,” 7.e., coloured impressions of luminosity. 
without form. | 
10 to 15 minutes. Fog no longer uniform, but “spotty” with patches of a 
brownish or greenish-bronze colour. 
18 to 21 minutes. A group of yellow or green luminous points—10 to 15 of 
them, arranged quincunx fashion, floats about, following the direction of the 
eyes. This phenomenon is, I believe, associated in some way with the yellow 
spot. It occurred in five out of the seven cases, lasting from 10 minutes to 
half-an-hour. 
23 to 28 minutes. Green ‘ dazzle-tints” predominate, beginning to break up, in 
some cases, into patches of green showing a fainter blue between. 
40 minutes. Blue increasing—green weakening and breaking up into isolated 
patches. | 
66 to 72 minutes. The luminous fog, which now shows no trace of green, begins 
to break up into blue patches, with occasional black spaces between. 
Generally after this, for a while it seems as if all the after-effects had gone. But 
on making a great effort of attention it becomes evident that they are still there. 
Swinging the arms, or going through dumb-bell exercises will bring them back 
for a time. 
106 to 120 minutes. Gradually a luminous violet fog seems to fill the 
surrounding space. For a minute or two it increases in brightness, and then breaks 
up near the middle in two or three places and seems to roll away on all sides. 
