210 Dr. G. J. Burch. | [ Mar. 7, 
Minimum Visible by Daylight, in Terms of the Minimum Visible after 
Two Hours in Total Darkness. 






Red. Blue-violet. Ratio. 
is} 1060°4 56 After setting up the apparatus. 
8:0 1070°0 134 After going about the laboratory. 
14°4 1064°3 74 After working in an adjoining room. 
14°0 :1064°7 76 Ee 5 
S}oN 1065°5 81 +} 5 A 
through an unknown angle after the record was pricked. At the end of the 
series of experiments the angles corresponding to all the records were read off. 
As it is important in all measurements depending on sensations rather than 
physical quantities to avoid mental bias, I did not, until these experiments 
were completed, refer to my notes taken in 1901 of the literature of the 
subject. It is the more interesting to find that,in Charpentier’s case, the eye 
increased in sensitiveness to white light at least a thousand times after a 
prolonged stay in darkness, and on one occasion two thousand five hundred 
times. ‘ 
It should be observed that, in my experiments, no brighter light was used 
than that of a very dull and cloudy December sky, and that only for a few 
seconds. Judging from other experiments, I consider that Charpentier’s 
highest figures must be well within the mark for a person coming from a 
fairly well-lit room in the summer time. 
But it would appear that by far the greater part of this increase of sensi- 
tiveness relates to the more refrangible rays. In my own experiments, the 
minimum visible for blue-violet varied in daylight from 1060 to 1070 times 
its value after two hours in darkness, whereas the range for the red is pro- 
portionally much greater, although the largest value is barely nineteen and 
the smallest eight times the minimum visible to the dark-adapted eye. This 
larger value was probably due to fatigue of the red sensation induced by the 
clow-lamp while adjusting the illuminaticn. 
Time-relations of the After-Images and Dazzle- Tints. 
I have reserved the description of the subjective sensations experienced in 
the dark room during these and other experiments, in order to discuss them 
separately. The notes of these observations were written at the time, in the 
dark, with pencil, ina small reporter’s note-book. A fresh page was turned over 
