SENSATIONS IN TERMS OF LUMINOSITY. 277 



electric light, the ordinates for which are given in ' Colour Photometry,' Part III. 

 The graphic results are shown in fig. 4. In this figure the hlue sensation curve 

 ordinates are on a scale 100 times larger than those of the red and green sensations. 

 The luminosity of this blue sensation is really very small, and except for the hue 

 would be negligible. 



The areas of the sensation curves in this figure, or of one constructed with the US, 

 GS, and VS, should, supposing the white light to be the same as that used in the 

 observations, be proportional to the constants in the colour equations for white light. 

 The areas of the US, GS, and VS curves are closely 1102, 529, and 247 on an empiric 

 scale, and these numbers, converted into percentages, give 



RS GS V W 



66-55 -f 31-96 + 1'49 = 100. 



The equation employed is 



RS GS V W 



68-42 + 29-90 + T68 = 100. 



Owing to the colour of the glass interposed in the beam, and to the slightly different 

 angular dimension on the retina of the images in the measurements on the two 

 occasions, this small discrepancy is fully accounted for. To see whether experiments 

 bear out this deduction, a measure was made under the conditions which were present 

 when the curves in ' Colour Photometry,' Part III., were made, and it came out as 



follows 



RS GS V W 



66-20 + 32-28 + 1'52 = 100. 

 This is sufficiently close to indicate that the measurements made are fairly exact. 



