Prof. S. P. Langley on Energy and Vision. 23 



second, it is certain that the eye, by the combined adaptability 

 of the iris and retina, can perceive lights whose intensities 

 vary in the ratio of 1 to 1 000 000000 000000* (10)^\ 



It will be understood that the writer does not profess any 

 competency in physiological optics, and that the preceding 

 observations, and the conclusions reached from them, are 

 both to be understood from the purely physical point of view. 

 This being premised, we will summarize the paper in the 

 following conclusions : — 



The time required for the distinct perception of an exces- 

 sively faint light is about one half second. A relatively very 

 long time is, however, needed for the recovery of sensitive- 

 ness after exposure to a bright light, and the time demanded 

 for this restoration of complete visual power appears to be 

 greatest when the light to be perceived is of a violet colour. 



The visual effect pi'oduced by any given constant amount 

 of energy varies enormously according to the colour of the light 

 in question. It varies considerably between eyes whi(;h may 

 ordinarily be called normal ones, but an average gives the 

 following proportionate results for seven points in the normal 

 spectrum, whose wave-lengths correspond approximately with 

 those of the ordinary colour divisions, where unity is the 

 amount of energy (about jqW ®^S) required to make us see 

 light in the crimson of the spectrum near A, and where the 

 preceding wave-lengths given correspond approximately to 

 the six colours violet, blue, green, yellow, orange, red : — 



Colour. Violet. Blue. Green. Yellow. Orange. Red. Crims. 



Wave-length.. ^^-40 '"•47 ''•53 '"-58 '"•60 '"•65 '"•75 



Luminosity ..1,600 62,000 100,000 28,000 14,000 1200 1 

 (Visual effect.) 



Since we can recognize colour still deeper than this crimson, 

 it appears from this that the same amount of energy may 

 produce at least 100,000 times the visual effect in one colour 

 of the spectrum that it does in another, and that the vis viva 

 of the waves whose length is 0'''75, arrested by the ordinary 

 retina, represents work done in giving rise to the sensation 

 of crimson light of O'OOOOOOOOOOOOS horse-power, or about 

 0*001 of an erg, while the sensation of green can be pro- 

 duced by 0-000000,01 of an erg. 



* It may be interesting to clieck tliis result by an entirely different 

 method. Ilie light of the sun is, according to Pickering, equal to that of 

 a star of — 2o'o stellar magnitude, or 4400,000000 times that of Sirius 

 (mag. — 1'4) which again is about 910 times that of a sixth magnitude 

 star, ordinarily considered the faintest visible to the naked eye. Here 

 the li ght of the sun is to that of the 7ninimu7n visibile as 1 to 4,000000,000000 

 (4x10'^), but the difference seems accounted for by the fact that the 

 ratio by this latter method is found for an eye exposed in starlight, by 

 the former for an eye in absolute darkness. 



