﻿308 Mr. P. G. Nutting on the 



distance in a given direction is determined in metre candles 

 and in watts per square centimetre ; from these total candles 

 per watt is found. Then by graphical integration of the 

 spectral energy and spectral luminosity (energy times visi- 

 bility) curves, Y m is readily calculated. 



For example, suppose that the spectral energy curve has 

 an area A, while the measured energy is W metre-watts then 



jErf\ = A=W. 



Call the area of the spectral luminosity curve B, and suppose 

 the illumination is metre candles, when and where the 

 energy is W metre-watts. Then 



Y m (~^dX = Y m B = C, 



"JV 



C A 

 By division, V m = -^- r r> , hence knowing C/W by direct 



determination and A/B by graphical integration, the funda- 

 mental constant Y m may be readily determined. The pre- 

 cision attainable by this method depends upon the uncer- 

 tainties in the three quantities u«ed, (i) relative visibility 

 Y/Y m , (2) the specific quality C/W, and (3) the spectral 

 energy E(X) of the source used in relative watts per unit 

 difference in wave-length. 



I have recently tried the monochromatic method with 

 mercury green light and the total spectrum method with 

 various sources. 



Filtered mercury light gave a very low value (4*86 metre- 

 watts to 12*7 metre candles or Vm=2 6 C/W) in spite of 

 every precaution to screen out the stray radiation and correct 

 for the remainder. 



Mercury light dispersed with a high intensity spectroscope- 

 gave better results, but the uncertainty is still large owing 

 to (1) the photometric comparison of pure green with white, 

 and (2) the removal of the thermopile (or other radiometer)? 

 from the spectroscope to face the energy standard. We 

 obtained 



220 mc. 6-54 mw. 38"6 C/W J. H. 



215 „ 38'0 N partly rested. 



320 „ 48-9 N rested. 



In measuring the brightness, merely looking at anything 

 illuminated with mercury li»ht will greatly depress the eye 

 sensibility to Hg green, while the fatigue ■ ansed by adjusting 

 the mercury la i p persists for perhaps an hour. Even using 

 a specially designed lamp and every precaution against stray 

 radiation, 1 regard the final result as uncertain by 10 per cent.. 



With the total spectrum method the sources used were 



