552 CAPTAIN ARNKY AND MA.TOR-GKNKRAL ! Ks'l'l M ; 



black and white were altered by trial to match when rotated in the patch of white 

 light formed by the recombination of the spectrum. When these same coloured discs 

 were rotated in day light or gas light they, as was to be expected, no longer formed 

 a grey, but had a predominant tint of green or red ; and the illuminating value differed 

 from that of the black and white discs. The alteration in hue and luminosity in 

 passing from one source of light to another, showed the necessity of using the same 

 source for making the match and for measuring the luminosity of the colours. In 

 adjusting the apparatus as explained above, with both parts of the receiving screen 

 white, we found that the rotating sectors had to be set with an aperture of 69 in 

 order to get a balance throughout the spectrum. Measurements were then made 

 throughout the spectrum of the intensity of light reflected from each of the coloured 

 cards, the aperture of the rotating sectors at each part giving the relative amount of 

 light reflected, the maximum value being 69. 



XXXIV. Calculations of Luminosity. 



The mean angular values of the coloured cards in the rotating disc which matched 

 the white and black disc were as follows : Emerald green, 1337 ; vermilion, 96'6 ; 

 French ultramarine, 129 0> 7. In order, therefore, to get the comparative amount of 

 light reflected from each coloured sector in the disc in terms of that reflected from the 

 emerald green, the readings of the red card were reduced in the ratio of 96'6/1337, 

 or multiplied by '722, and those of the blue by 1297/1337, or '97, those of the 

 green card being unaltered. From these figures the curves on fig. 16 were plotted ; 

 the straight line at 69 being taken to represent the amount of light at each part of 

 the spectrum which was reflected from a white card sector of 1337, the ratio of the 

 ordinates of the other curves to 69 would indicate the proportion of each ray reflected 

 from the coloured sector as compared with that from a white sector of 1337. 



From these curves the luminosity curves in Plate 20, fig. 17 were constructed. 

 The outer curve is the normal curve of white light, as given iu Part I. of " Colour 

 Photometry ' ( VIII. and fig. 3), the scale of the spectrum being the same. The curves 

 for the colours were then made, their ordinates bearing the same proportion of those of 

 the outer curve that those of the curves in Plate 20, fig. 16 bear to 69. 



