376 8. P. Langley — Energy and Vision. 



Fraction of standard green light (A= 0^-55) required for certain 

 vision =0-000655 X 0'033 X 0'000003 = 0-000000,0000655. 



Fraction of standard scarlet light (\ = fi, 65) required for cer- 

 tain vision =0-00235 X2'X 0-000003 = 0-000000,0141. 



Fraction of standard crimson light (A= 0^-75) required for cer- 

 tain vision = 10'X 0-000003 = 0-00003. " 



The measures were made on July 3d and 11th, the sky be- 

 ing a fairly good milky blue and the sun within one hour of 

 the meridian. 



Assuming that the energy per millimeter of the standard 

 spectrum was 0-000001 calorie per half second for the wave- 

 lengths 0^-55 and 0^-75, we have from table I : 



For A= 0^-40, energy = 5'3-4-(20'7Xl, 000000) calorie. 

 " A=0 /z -65, '" =22-2-^(20-7x1,000000) " 



by means of which, we reduce each of the above values to 

 absolute measure, obtaining for the maximum value of the 



Minimum Visibile. 



Reciprocal of Reciprocal of 



Violet.... 0^-40 63,000000,000000 calories= 1,500000 ergs. 



Green 0^-55 15000,000000,000000 " =360,000000 " 



Scarlet .__ 0^-65 66,000000,000000 " = 1,600000 " 



Crimson .. 0^75 33000,000000 " = 780 " 



Stating these values in terms of horse power we have 



Min imum Vis ib He. 



h. P . 

 Violet 0^*40 0-000000,000000,00018000 



Green... 0^55 0*000000,000000,00000075 



Scarlet _ _ _ 0^-65 0*000000,000000,00017000 



Crimson (near A) . _ 0^75 0-000000,000000,34000000 



The measurement of the minimum visibile is subject to 

 variations of a much wider range than those of the photo- 

 metric method and may perhaps be in error by 100 per cent* 



* The relative sensitiveness of the eye of the observer in question (F. W. V.) 

 for the extreme red or violet, as compared with its power of detecting green light, 

 appears to he somewhat less when determined by the method of minimum visibile 

 than by the reading of fine print. 



By the former we have 



