1891.] The Numerical Registration of Colour. 231 



the colour of this last patch is approximately the same hue as that of 

 the glass. 



In the path of the reflected beam, but between the prism and the 

 silvered mirror, is inserted a piece of plain glass, which can be made 

 to reflect part of the white beam into the monochromatic patch of 

 light, a square patch of this white light being formed by means of a 

 third lens. We thus have monochromatic light mixed with white 

 light. The requisite intensity of the added white light can be 

 adjusted by means of rotating sectors which open and close at will 

 during rotation, and the integrated luminosity of the mixed beame 

 can be altered by this, together with the adjustable slit in the slide. 

 The slit may probably have to be moved in the spectrum, to make the 

 hue of these mixed lights the same as that of the glass, but by trial 

 the position of the ray, whose colour, when diluted with white light, 

 makes the match, is readily found. The position of the slit in the 

 spectrum is noted, as also the aperture of the sectors. The relative 

 luminosities of the beam reflected from the plain glass mirror and of 

 the coloured ray are next measured by placing a rod in the path of 

 the two beams, and equalising by the sectors the luminosity of the 

 shadows which are illuminated, the one by the spectral ray, and the 

 other by the white light. When the sector aperture is noted the 

 registration is complete, as far as hue is concerned, but the luminosity 

 of the ray transmitted through the glass should be compared with 

 ^hat of the unabsorbed reflected beam, and then the total luminosity 

 is doubly recorded. 



Should the colour of a pigment be in question, the ray reflected 

 from the silvered mirror is made to fall on the pigmented surface, 



d the same procedure adopted. 



Should a purple glass (say) have to be registered, we proceed in a 

 ilightly different manner, the patch of colour light passing through 

 the purple glass is superposed over the spectrum patch, and the slit 

 in the slide is moved till a ray is found which will make white liyht 

 when superposed on the colour of the glass. The luminosities of this 

 white light, of the reflected beam, and of the spectral colour, are 

 Compared inter se, and there are then sufficient data on which to 

 make numerical registration. In the paper which will be submitted 

 to the Royal Society, and of part of which this is a preliminary note, 

 the details of registration will be entered into more fully. 



The signal glasses having to be used at night with oil or gas, their 

 hue must be registered in these lights. As the spectrum colours are 

 always the same, it is convenient to use the electric light spectrum, 

 and the only alteration in the apparatus is to use two gas lights to 

 illuminate two square apertures, in front of one of which the glass, 

 ose colour has to be measured, is placed. The images of these 

 ures are thrown on the screen, the coloured image touching the 



OL. XLIX. K 



