756 ENERGY AND VISION 



This particular paper was less transparent for the extreme red 

 than for the green, a fact which had to be taken into account. 



Measurements. — Eighteen persons were examined; two series of 

 experiments were performed: one after 8 minutes in the dark, and 

 one after 25 minutes. Only five persons were examined for all wave 

 lengths. The others were merely tested for the radiation 0.55. The 

 measurements were carried on in the following way: When nine 

 sheets of paper were placed exactly against the slit, generally no 

 light could be seen. Then, one after the other, the sheets were re- 

 moved, according to the intensity of light, and usually, owing to the 

 relatively large area (0.1 sq. cm.), there was no difficulty whatever 

 in determining the order of magnitude of the minimufn visibile. 

 Namely, one sheet added gave a black impression, and this sheet 

 removed left a \'isible, although very faintly colored, image of the 

 slit. We sought, as Langley did, to determine the minimum visibile, 

 defining this to be, not the smallest light whose existence it is possible 

 to suspect, or even to be reasonably certain of, but a light which is 

 observed to vanish and reappear when silently occulted and restored 

 by an assistant without the observer's knowledge (Fig. 4). 



On top of the last sheet of paper, another slit was placed, across 

 the first one. Its jaws were cut in such a way that a square opening 

 was left between them, (see Fig. 5); thus a square or rectangular 

 figure was delimited by four moving lines. At first, the slit was ad- 

 justed so as to cut a little window of 1 sq. mm. on the luminous spot. 

 If the window could not be seen, the jaws of the slit were moved 

 micrometrically until the spot became visible. The maximum open- 

 ing corresponded to a vertical motion of 5 mm. 



It was found that most women generally require more time than 

 men to reach the same degree of sensitivity. Most of the men tested 

 became adapted in 5 minutes (viz., could see a light corresponding 

 to an energy of the order of magnitude 10~^) while it required 15 

 to 20 minutes for women to see the same thing. Moreover, some 

 of them could only see the spot spasmodically appearing and dis- 

 appearing, while men had a continuous impression. Only an increase 

 in the intensity of about 50 to 100 per cent was able to give them 

 the same visual impression. As will be seen, the differences vary 

 between and 25 per cent among men for a given wave length; 



