Luminosity of Coloured Surfaces used for Colour Discs. 119 



of the spectrum, a greenish-yellow, progressed in luminosity at the 

 same rate as white light. Thus, if part of a white screen were 

 illuminated by this colour and another part by white light, and the 

 luminosities were equal (say) to one candle, then if the two beams 

 were equally diminished they would still match in luminosity until the 

 light was so feeble that it ceased to stimulate the retina. Other rays 

 lying not far from this ray, both on the red and green side of it, gave 

 practically the same results. When, however, the red was compared 

 with the white, each being made equal (say) to one candle, equal 

 diminution of the beams did not show the luminosities as the same, 

 the red becoming rapidly less luminous than the white. With the 

 blue-green, the blue, and the violet the reverse was the case, the 

 white becoming darker than the colour as the beams were equally 

 diminished. 



A more extended research which is nearly complete shows that the 

 observations recorded in Part III of " Colour Photometry " are correct 

 and can be applied to the problem which I wished to solve. 



Further, it was shown in the same paper that colour disappeared 

 from all rays of the spectrum long before (except in the case of the 

 pure red) their light was extinguished, this last owing to the feeble 

 stimulation of the retina. Naturally, as the colour began to disappear, 

 the matching of the luminosity of the ray under consideration with 

 that of white became easier to carry out. 



These facts made it possible to devise a ready method to ascertain 

 the luminosity of any colour. If we take two yellow discs, one (say) 

 8 inches in diameter and the other 4 inches, and between them sand- 

 wich a pair of interlaced black and white discs of 6 inches diameter, 

 and rotate the four discs on a rotating machine at a speed which will 

 make the black and white into a grey without scintillation, this 

 grey can be made, by altering the proportion of black to white, to 

 match the luminosity of the yellow. A very exact match can be 

 obtained by observing the discs through a black transparent medium, 

 such as the black obtained on a photographic plate after development 

 with methol or amidol developers. The deposit may be so dense that 

 the yellow colour may practically disappear, and the two dull greys 

 may then be readily matched. The luminosity of the yellow in terms 

 of the white is given by the angle which the white subtends when the 

 small proportion of white reflected from the black annulus is added 

 to it. 



The same procedure may be adopted for a green colour and its 

 luminosity be obtained. It may be stated that four or five observa- 

 tions for each colour should be made if great exactness is required. 



When the luminosities of these two colours have been determined, 

 4-inch discs of them may be interlaced with a blue, and a grey formed, 

 which can be matched with a grey formed of black and white as before. 



K 2 



