1894.] Measurement of Colour produced by Contrast. 223 



lens, also on the cube. A thin rod f inch diameter placed in the 

 paths of the two beams caused two shadows to be cast on the 

 cube, one illuminated by pure white light and the other by the 

 spectrum colour. These were separated from one another by an 

 interval illuminated by a mixture of the spectral colour and white 

 light, and on each side of the shadows the same diluted colour was to 

 be found. The appearance of the side of the cube was as below. 



A B 



A was a stripe of white light, B of colour, c c c of the same colour 

 dilated with white. The intensity of the D sodium light thrown on 

 the surface was 0'5 of an amyl acetate lamp at 1 foot, the intensity of 

 the other colours can be obtained from the luminosity curve in the 

 4 Phil. Trans.,' " Colour Photometry," Part III, 1892. 



The patch of colour from instrument No. I was thrown on the 

 face of a cube (No. II) I foot away from the first cube, and was used 

 to match the contrast colour produced on A, fig 1. The beam of white 

 light also fell on the same face of the cube, and the intensities of each 

 could be altered at will, that of the colour by opening or closing the slit 

 through which the colour came, and that of the white light by rotat- 

 ing sectors. By this means any dilution of colour could be secured. 

 It may be mentioned that the effect of using a strip of the face of the 

 cube equal in width to the width of A was tried, but no advantage 

 was found by so doing. The method of procedure was as follows : 



With instrument No. II the colour to be used and the white beam 

 were thrown on the face of the cube No. II. The luminosities of 

 the two were made as nearly equal as possible. With instrument No. I 

 a colour, which it was judged was nearly the dominant colour of the 

 contrast colour on A, was thrown on the face of the cube No. I 

 and white light added. When it was found that a match was per- 

 fected by slight changes in the colour and in the intensity of the 

 added white, the scale No. of the colour was read, from which the 

 wave-length would be determined, and the relative luminosities of 

 the white and the colour measured directly. The aperture of the slit 



