their Relation to certain other Visual Phenomena. 275 



white light. All the various components set up fatigue after a 

 moment's action, but green more than the others ; if, therefore, the 

 green stimulus were diminished to an extent corresponding to the 

 excess of fatigue which it produced, the tint of the pulsative image 

 might be expected to become neutral, like that of the ordinary nega- 

 tive after-image. Different parts of the green portion of the spectrum 

 were accordingly cut out by interposing strips of black card of various 

 widths, and it was found that when the green rays from A 5030 to 

 A 5 -170 were intercepted, the tint of the pulsative image was absolutely 

 neutral. 



White compounded from Red and Blue-green. — Such a white always gave 

 a pink pulsative image — a fact which confirms the inference derived 

 from previously described observations that the blue-green sensation 

 is, after an interval of repose, more readily fatigued than the red 

 sensation. 



White compounded from Yellow and Blue. — A white colour-patch was 

 formed by combining a blue of A 4530 to A 4710 with a yellow of A 5650 

 to A 5860. The colour of the pulsative image was rather doubtful, but 

 an artist (who did not know what to expect) unhesitatingly pro- 

 nounced it to be yellow. Since the Young-Helmholtz theory supposes 

 that yellow excites the green sensation, this result was unexpected. It 

 is also opposed to the usually received opinion that the sensation of 

 yellow is more readily exhausted than that of blue.* 



V. Pulsative Images of Complete Spectrum. 



The spectrum was projected upon a screen covered with white card- 

 board, which was put in the place of the slit-screen, as shown in fig. 8. 



Fig. 8. 



L 



The beam of intermittent white light was reflected upon the screen by 

 means of a mirror and formed an oblong bright patch upon the site of 

 the spectrum. The upper part of the mirror was covered by a screen, 

 so arranged that the site of the spectrum was longitudinally divided 

 into two equal parts, the lower of which was exposed to intermittent 



* Foster, loc. cit. 



