378 S. P. Langley — Energy and Vision. 



investigated. The method was an electrical one. There was 

 automatic registration on a chronograph of the instant of exhi- 

 bition, and determination of the instant of response as the ob- 

 server pressed a key. The interval of course includes quite a 

 train of distinct operations. According to Mendenhall (this 

 Journal, III, vol. ii, p. 156), that portion of the action of brain 

 nerve, and muscle which produces the mechanical effect, and 

 which may be called automatic, takes place in certainly but 

 little over one-tenth of a second. But the sensations which 

 demand a conscious concentration of the attention, and espe- 

 cially those which require for their registration a decision of 

 the judgment, occupy an interval several times as great. The 

 perception of a light just at the verge of visibility probably 

 involves an exercise of judgment, — an answer to the question, 

 " Do I see the light or do I not ?" — although the question may 

 not be consciously propounded, and accordingly this kind of 

 perception may be included in that class of combined sensa- 

 tion and mental operation which involves a choice. Professor 

 Mendenhall found for the time required to decide between red 

 and white sec - 443 and to decide between a circle and a triangle 

 Qsec. -494. w e have found for the average of over 1000 observa- 

 tions of the disappearance or reappearance of a very faint 

 light (perhaps 20 times as bright as the faintest perceptible), 

 0sec. -5Q^ b u t corresponding measures with a moderately bright 

 spectrum, the light being about 10,000 times as intense as that 

 called " very faint," gave 8ec - '242, a number which is inter- 

 mediate between the times found by Professor Mendenhall for 

 the appearance of a white card (0 sec - *292) and that of an electric 

 spark (0 sec< -203). We may therefore conclude that distinct 

 vision for a very faint light demands about one half second of 

 time, while the perception of light of ordinary brightness re- 

 quires only about half that interval. It is possible that differ- 

 ences in the rapidity of the perception for lights of different 

 colors might be detected on more exhaustive study, but none 

 have been noted in these experiments other than those which 

 were attributable to the variation of intensity. 



It will be seen that quantitative measures of the effect upon 

 the eye of different rays whose luminosity varied in the pro- 

 portion of 200 000 : 1, were actually obtained and that it 

 would have been possible to considerably exceed these limits, 

 especially when it is considered that the photometric measures 

 were confined to lights of feeble intensity. Since it is possible 

 to look directly at the sun for as short a time as one-half sec- 

 ond, it is certain that the eye, by the combined adaptability of 

 the iris and retina, can perceive liohts whose intensities vary in 

 the ratio of 1 to 1 000 000000 000000.* (10; 16 



* It maybe interesting to check this result by an entirely different method. 

 The light of the sun is, according to Pickering, equal to that of a star of —25*5 



