their Relation to certain other Visual Phenomena. 273 



was produced by mixing- red and green rays. An orange-yellow, 

 made by combining the spectrum rays from the extreme red to 

 X 5340 in the green, had a slate-coloured or nearly neutral pulsative 

 image; the addition of a very little more green turned the image 

 pink. 



Green. — A colour-patch sufficiently illuminated by green rays taken 

 from any part of the spectrum between greenish-yellow and greenish- 

 blue inclusive (about X 5750 to X 5050) produced a pink or dilute purple 

 pulsative image; the purple was strongest when the exciting colour 

 was a full green, but it never reached an intensity equal to that of 

 the blue-green excited by red when the conditions were most favour- 

 able. On the other hand, there can be no question that a purple 

 pulsative image after green is much more easily produced than a blue- 

 green one after red, a fact which tends to indicate that, at least after 

 a short period of repose, the colour-sense organs become fatigued more 

 quickly by green light than by red. It seems to be generally believed 

 that the red sensation is more readily exhausted than the green. * 

 Kood,f however, attributes the "well-known intolerance of all full 

 greens to the fact that green light exhausts the nervous power of the 

 eye sooner than light of any other colour," this exhaustion being 

 "proved by the observation that the after-pictures . . . are more 

 vivid with green than with the other colours." The results of my 

 own observations lead me to think that while after a prolonged gaze 

 at brightly illuminated colours, blue-green after red is more con- 

 spicuous than purple after green, the opposite may be the case when 

 the exposure has been brief or the illumination feeble. In the case of 

 the pulsative image, however, account must be taken not only of 

 fatigue, but also of persistence and of the latent period during which 

 the first impact of light upon the eye fails to produce any recognisable 

 sensation. 



Blue. — Though the ordinary after-image of blue is orange, the 

 pulsative image upon the screen was generally seen as some form of 

 impure purple, variously described as dull pink, salmon, or flesh colour. 

 The same was often the case when the eyepiece methods were employed. 

 Among the blues tested were a mixture of X 4700 to X 4950, and one 

 of X 4550 to X 4760, besides many others of which the limiting wave- 

 lengths were not determined. By Method II a good orange-yellow 

 image could always be produced from the last-named blue, provided 

 that the illumination was sufficiently strong and the various lumi- 

 nosities carefully adjusted. 



Blue-violet and Violet. — The ordinary after-image is yellow. The 

 screen method showed scarcely any image at all for light of wave- 

 lengths less than about A. 4500. With the eyepiece methods the image 



* Foster, ' Text-book of Physiology,' 6th edition, p. 1382. 

 f 'Modern Chromatics,' 2nd edition, p. 295. 



