S. P. Langley — Energy and Vision. 379 



It will be understood that the writer does not profess any 

 competence in physiological optics, and that the preceding ob- 

 servations 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 sensitiveness 

 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 color. 



The visual effect produced by any give?i, constant amount 

 of energy varies enormously according to the color of the light 

 in question. It varies considerably between eyes which may 

 ordinarily be called normal ones, but an average gives the fol- 

 lowing proportionate result for seven points in the normal 

 spectrum, whose wave-lengths correspond approximately with 

 those of the ordinary color divisions, where unity is the 

 amount of energy (about y^Vo" er g) required to make us see light 

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

 preceding wave-lengths given correspond approximately to the 

 six colors violet, blue, green, yellow, orange, red. 



Color. Violet. 



Blue. 



Green. 



Yellow. 



Orange. 



Eed. Crims. 



Wave-length, ^40 



^•47 



^•53 



'"•58 



'"•60 



^•65 *»« 



Luminosity, 1,600 

 (Visual effect.) 



62,000 



100,000 



28,000 



14,000 



1200 1 



Since we can recognize color still deeper than this crimson, 

 it appears that the same amount of energy may produce at 

 least 100,000 times the visual effect in one color of the spec- 

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



waves whose length is O^^, arrested by the ordinary retina, 

 represents work done in giving rise to the sensation of crim- 

 son light of 0-0000000000003 horse power, or about 0-001 of 

 an erg, while the sensation of green can be produced by 

 0-000000,01 of an erg. 



ptellar 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 light of the sun is to that of the minimum 

 visibile as 1 to 4.000000.000000 (4xl0 1-2 ), 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. 



