274 Dr. S. Bid well. On Negative After-ipiages, and 



usually appeared as a pale bluish-pink, which could be closely matched 

 by blue-violet diluted with much white. The persistence of violet 

 impressions is very great, and it is not unlikely that the bluish-pink 

 image was due merely to the intermingled action of the violet and 

 white-light rays (as in a Maxwell's disc), and was not a true pulsative 

 after-image. In the circumstances mentioned in the last paragraph, 

 when blue light gave an orange pulsative image, blue-violet also gave 

 a yellow one, the persistence of blue-violet being less with strong than 

 with weak illumination. 



Purple. — A bright purple was made by combining red, X 6180 to 

 X 6810, with blue-violet, X 4330 to A 4420. The ordinary after-image 

 of the red alone was blue-green, and that of the purple grass-green. 

 The pulsative image of the purple formed on the screen was, however, 

 blue-green, and when the slit admitting the blue-violet light was 

 alternately covered and uncovered, no change in the colour of the 

 image could be detected. 



IV. Pulsative After-image of JFIiite, 



Recombined Spectrum. — If the slit-frames and their appurtenances are 

 removed from the slit-screen I, fig. 3, the whole spectrum is recom- 

 bined by the lens K, and forms upon the screen L a white " colour- 

 patch," the illumination of which can be varied in a known manner by 

 changing the width of the collimator-slit. The illumination of the 

 " white-light disc " (which, during an experiment, alternates with the 

 white " colour-patch ") can also be adjusted to certain known intensi- 

 ties. A large number of experiments, which need not be described in 

 detail, were made with various illuminations of the white colour-patch 

 and of the white-light disc. The colour of the pulsative images of 

 the white patch, which is not in general neutral like that of the ordi- 

 nary [after-image, was found to depend not only upon the absolute 

 values of the two illuminations but also upon their ratios. Broadly 

 speaking, it may be stated that with feeble illumination the patch 

 appeared yellow (probably only an effect of weak intermittent light), 

 w T ith very strong illumination it was a neutral grey, and with all such 

 intensities of illumination as are ordinarily employed it appeared a 

 more or less decided purple. 



In my former paper reference was made to the purple tint assumed 

 by a white card when seen through the original black and white disc, 

 and a distinguished physiologist, who saw the effect, expressed the 

 opinion that the colour might be due to the "visual purple." In the 

 light of the observations described in the present paper, it seemed 

 possible that the phenomenon might be explained by the hypothesis 

 that the purple was really an after-image of the green component 

 which, according to the Young-Helmholtz theory, is contained in the 



