1891.J Colour of Cases of Tobacco Scotoma, 



493 



the colour-patch apparatus described in " Colour Photometry," in the 

 Philosophical Transactions,' 1886, by General Festing and myself. 

 The objects first in view were to test his perception of the spectrum 

 colours, and then his retinal field colour perception for the same. 

 A template was cut out after the manner described by General Festing 

 and myself in the second part of " Colour Photometry " (' Phil. 

 Trans.,' 1889), of such a shape that all the spectrum lying between 

 X 4600 and X 6600 was reduced to equal luminosity when it was ro- 

 tated in front of the spectrum. Diaphragms containing holes of 

 different sizes were placed in front of the last prism, and thus a round 

 spot of monochromatic light of the same luminosity was produced 

 upon the screen when a slit was passed through the spectrum. From 

 the red end to X 5270 he called the whole of the colours white, and 

 from that point he began to see blue, called the colours bluish and 

 blue. When the full illumination for all the colours was used, the 

 same results were obtained. From this examination it would appear 

 that he was totally deprived of the sensation of any colour except of 

 blue. A subsequent examination of his perception of the luminosity 

 of different rays, however, has to be taken into account, for in the 

 first examination he had no light of pure white with which to compare 

 the colours. In the next experiments, a strip of white light was 



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