Colour Adaptation. 



Ill 



IV. The fourth method is comparing the appearance of colours in a photo- 

 meter, one colour being illuminated by daylight, and the other by artificial or 

 coloured light. The objects are then examined first by daylight, and then 

 by the artificial light which lias been used. The difference between the 

 results obtained in this way and those of the photometer represents the 

 effects of colour adaptation. The same colours are also examined in the 

 photometer, both sides being illuminated first by daylight and then by 

 artificial light. 



When a spectrum is examined after the eyes have been exposed for 

 20 minutes in a room illuminated only by sodium light, the yellow appears 

 to have disappeared from the spectrum. The red and green appear to meet 

 without any intermediate colours, and the red, orange and green have lost 

 any yellow character which they previously possessed. There is no increase 

 in the blue or violet, and the red and green are not diminished. If, before 

 exposing the eyes to the sodium light, a small portion of terminal red be 

 selected, this is found to be just as visible after the exposure as before. 



The same condition is found with artificial light in which the yellow rays 

 predominate. Yellow is discriminated with difficulty from white by electric 

 light (" Osram " incandescent). It is often impossible to detect a yellow stain 

 on a white cloth by this electric light which is very obvious and marked by 

 daylight. 



When blue-green spectacles are first put on all white objects appear a vivid 

 blue-green. This blue-green gradually fades until, in about 10 minutes' 

 time, a piece of white paper or white cloth appears absolutely white, 

 without a trace of blue-green. In fact, though I know that blue-green 

 light is falling upon the eyes, I can see no trace of this colour. This 

 shows conclusively how very little the conscious judgment contributes to 

 these results, apart from the perception of relative difference. If the sky 

 be white, misty, and overcast, this will appear only faintly coloured blue- 

 green ; if it be much brighter, or a naked filament of an electric light be 

 looked at, these are seen as blue-green. Black objects appear black throughout. 

 I have never been able to find the faintest tinge of red in a black object. 

 When the spectacles are removed white objects appear a decided rose pink, 

 and a perceptible interval elapses before objects regain their normal colour. 



I found that I could read all Stilling's pseudo-isochromatic tables for 

 testing for colour-blindness with the blue-green spectacles on. An examina- 

 tion of the spectrum immediately after putting on the blue-green spectacles 

 showed that there was no red to be seen, there was a small amount of 

 orange, and the yellow, green, blue, and violet were visible. After wearing 

 the glasses for about 10 minutes until white appeared white, and then 



