560 CAPTAIN ABNEY AND MAJOR-GENERAL FESTING 



XXXVII. Photograplis of the Rotating Discs. 



The next experiment made was to ascertain if the effect of equality of grey 

 produced on the eye when the coloured and black and white sectors were rotated 

 would be shown if they were photographed together. 



The above figure shows the impression produced on a photographic plate by 

 rotating green, red, and blue sectors, which were kept of the same angular value 

 respectively as given in XXXIII., as were also the white and black sectors. They 

 were illuminated by the electric light, and photographed on a bromo-iodide plate. It 

 will be seen that there is a falling off of luminosity in the combination of the coloured 

 sectors, an effect which might have been predicted from the Curve V. in fig. 16, in 

 which there is a falling off of intensity in the violet. This shows itself in the photo- 

 graph, since the plate is but little sensitive below F towards the red. 



XXXVIII. Matches by Colour-blind People. 



This experiment suggested that it would be of great interest to try what results 

 would be obtained by a colour-blind person using the same three sectors. R., who 

 had so kindly helped us before (see " Colour Photometry," Part L, XVI.), again 

 came to our aid and made observations. He was totally deficient in the perception of 

 red, and mistook the vermilion disc for dark green when we showed it to him. The 

 total absence of red perception in him enabled him to match green and blue rotating 

 sectors against black and white sectors. He obtained a balance when the blue sector 

 was 115, the green 245, and the white sector when corrected 134'8. Since the 



