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



Fraction of standard green light (X,=0'^'55) required for 

 certain vision 



= 0-000655 X 0-033 x 0-000003 = 0-000000,0000655. 



Fraction of standard scarlet light (\ = (f'Qh) required for 

 certain vision 



= 0-00235 X 2- X 0-000003 = 0-000000,0141. 



Fraction of standard crimson light (X, = 0*^-75) required for 

 certain vision 



= 10- X 0-000003=0-00003. 



The measurements were made on July 3rd and 11th, the 

 sky being a fairly good milky blue and the sun within one 

 hour of the meridian. 



Assuming that the energy per millimetre 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 X=OMO, energy= 5-3-4-(20-7 x 1,000000) calorie 

 „ X=0'^-65, „ =22-2-^(20-7 x 1,000000) „ 



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

 absolute measure, obtaining for the maximum value of the 



Minimum Visihile. 

 fjL Reciprocal of Reciprocal of 



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

 <>reen ...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 



Minimum Visihile. 



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 minimutn visihile 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 tlie eye of tlie observer in question 

 (F. W. V.) for the extreme red or violet, as compared with its power of 

 detecting green light, appears to be somewhat less when determined by 

 the method of minimum visihile than by the reading of fine print. 

 By the former we have : — 



Sensitiveness, violet . .{^^0) : green ('^•55) =1 : 240. 

 „ scarlet. .(,ul*65) : green ( -55) =1 : 230. 



„ crimson (y-'lb) : green ('^•55) = 1 : 450,000. 



