862 Mr. H. E. Ives on the 



Discussion. 

 Use of Luminosity Curve. — To the standard conditions of 

 heterochromatic photometry which have been outlined above 

 must now be added the use of the average eye as defined by 

 the data of Table II. Upon inspection of the curves of 

 fig. 3, it is clear that the individual variations are of such 

 magnitude as to prevent the use of more than a very few 

 eyes for coloured light photometry where any marked colour 

 difference exists. The extreme observers, for instance, 

 would measure a red light against a " white " light with a 

 difference of 100 per cent. These differences become very 

 much less when the light sources under comparison are any 

 of the ordinary illuminants, but some means of obviating 

 them is necessary in order to reach a complete solution of 

 the photometric problem. An approximation to the condition 

 of using a normal eye can of course always be reached by 

 securing the cooperation of a large number of observers, and 

 until each standardizing laboratory is prepared to obtain 

 luminosity curves of its observers and correct their photo- 

 metric work accordingly, this is the procedure to be recom- 

 mended. 



The most rational manner of approximating to the average 

 eye is, however, by ascertaining the luminosity curve of each 

 observer in the laboratory and calculating the correction to 

 be applied to his results for the energy distribution under 

 measurement. According to the results of this investigation, 

 the total light is accurately the sum of the parts as measured 

 under the standard photometric conditions. It should there- 

 fore be possible to apply known energy distributions to the 

 observers' luminosity curves for certain physical conditions 

 of relative intensity (such, for instance, as equality at *58 /j,), 

 and then from the relative areas of standard and test-lamp 

 luminosity curves in the case of the observers' eyes and in 

 the case of the average eye to obtain the correction which 

 would bring the observers' results to average. 



This mode of correction was accordingly tested. A 

 " 4 watt " carbon lamp was measured on the spectrophoto- 

 meter, and by the above method its visible energy distribution 

 was found to correspond to that of a black body at 2080° abs. 

 The normal equal-energy spectral luminosity curves of the 

 two observers which varied most from the normal (14 

 toward red, 10 toward blue) were then multiplied at each 

 wave-length by the energy values for the 4 watt lamp (BB at 

 2080°), and for the tungsten lamp at 1\15 w.p.c. (BB at 2360°), 

 the latter values being taken arbitrarily as unity at '58 fi. 

 The areas of the four curves, measured by a planimeter, 



