550 



CAPTAIN ABNEY AND MAJOR-GENERAL FESTINO 



Luminosity Curves reduced to the Normal Scale. 



XXXV. Testing the Accuracy of the Measurements. 



Let us now consider fig. 16. The space between the horizontal line at 69 and that 

 at the bottom of the diagram may be taken to represent white light, and that between 

 any other two horizontal lines would, in the same way, represent degraded white or 

 grey. 



Evidently also the space between the combination curve No. IV. and the bottom line 

 would represent grey, for it has been shown that, if all parts of the spectrum be com- 

 bined in proportion to the ordinates of that curve, the result is grey. If now a 

 horizontal line be drawn tangential to the highest point of the curve, the space 

 between that and the bottom line would represent grey ; and, as that between the 

 curve and the bottom line represents grey, the difference of these two or the space 

 between the curve and the tangential line must also represent grey : that is to say, 

 grey should result from the combination of all parts of the spectrum in the proportions 

 of the ordinates lying between the tangent and the curve. 



This consideration suggested a good test for the accuracy of the method employed 

 in the measurement of the colours, and of the proportion in which they should be 

 combined to produce white (or grey). If the rays of the spectrum itself could be 

 taken in the proportions in which they are reflected from any pigment, say emerald 

 green, and recombined, the resulting light should be of the same colour as that 

 reflected from the pigment. This was proved to be so in the following way : A card 



