ON COLOUR PHOTOMETKY. 



559 



In this case the chrome yellow was taken as the standard. The chrome yellow 

 required 169 of the disc, and the blue 191; hence, the ordinates of the blue were 

 multiplied by 1'13. Curves I. and II. in tig. 21 (Plate 21) give graphically the 

 intensities of these two colours, as also Curve III. the sum of the intensities. It will 

 be seen that there is a deficiency in the yellow and in the blue and violet, which 

 together will give a grey, as indicated before. Fig. 22 shows the luminosity of the 

 colours in the spectrum of the light from the positive pole of the electric light. 

 Coming to the question of the total luminosity of the two sectors, we have the area 

 of the chrome yellow = 464, whilst that of the blue = 93. The sum of the two is 

 557. As the angular value of the yellow sector is 109, this value has to be reduced 

 by 169/360 = '47, and is 261 7. 



The angular value of the white sector used in the match discs was 158 '5. As the 

 black reflected '0833 of white light, the total value of the rotated white was 

 (158-5 + iJOl'5 X '0833, or) 175'4. The area of the curve of luminosity of the white 

 being 534, the luminosity of the grey was 175 -4/360 X 534 = 260'4, a value very 

 near to that found as that of the rotating coloured sectors. The fact that we are 

 only, in this case, dealing with two colours, and that these colours are fairly luminous, 

 makes the calculated and observed values of the greys in the two discs less liable 

 to differ than when the colours are more in number and of less luminouty. 



