ON COLOUR PHOTOMETRY. 
559 
§ LI.— Relative Luminosity of Rays for Different Spectrum Intensity. 
Having found that the curves of luminosity of a spectrum when feeble and when 
bright differed, it became a matter of some importance to ascertain in what manner 
the relative luminosity of the rays varied when the intensity of the light which 
formed the spectrum was altered in a definite ratio. Evidently the most satisfactory 
method of ascertaining this was to throw a patch of white light on the screen and 
then to diminish its luminosity to known amounts, and having selected some ray of 
the spectrum, to equalize their luminosities. The box already described (fig. 36) was 
brought into requisition, and a beam of white light was caused to illuminate one half 
of the white patch on the screen at the end of the box, and the other half was 
illuminated by the ray whose luminosity was to be tried. Rotating sectors were 
placed in each beam ; the apertures of those in the white were fixed at different 
angles, whilst those of the sectors in the coloured beam were opened or closed till 
the luminosities appeared the same to the eye, a series of readings being taken for 
each ray. The results thus obtained were plotted, and some typical ones are shown 
Fig. 46. 
k, 0 10 20 30 - 40 50 60 - 70 80 90 100 NO 120 130 140 150 160 170 180 
-v Degrees of sector aperture in- white beam. 
Relative luminosities of rajs with different intensities of the spectrum. 
in fig. 46. The ordinates are the apertures of the sectors in the monochromatic rays, 
and the abscissse the apertures of the sectors in the white beam. The tangent of 
the inclination to the vertical of the curve at any point, therefore, represents the 
ratio of the luminosity of the coloured to that of the white beam for a certain 
intensity of light. If this ratio were the same for all intensities the curve would 
become a straight line starting from the origin. This is the case, it will be seen, 
with one ray only, that at scale number 46’3, or about X 5618. This ray and 
