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Dr. S. Bid well. On Negative After-images, and 



small angle, a white patch of any desired luminosity appearing in it's 

 place. The hues of the negative images seen upon the white patch 

 are often very different from those of the pulsative images formed 

 when the disc is rotating continuously. 



III. Pulsative Images due to Various Colours. 



Red. — A reel colour-patch formed on the screen by a combination of 

 rays extending from the extreme limit of the spectrum to X 6450 gives 

 no pulsative after-image at all, the white-light disc, whatever may be 

 its intensity, appearing white throughout. If the slit is further opened 

 to admit rays up to X 6320 a faint blue-green image is seen upon the 

 white-light disc, provided that the latter is not too strongly illumi- 

 nated ; with apertures greater than No. 5 of the diaphragm 0, fig. 3, 

 the blue-green image disappears. The absence of a pulsative image 

 after a low red is, no doubt, in great measure due to the superior 

 persistence of this hue, for the ordinary after-image is quite distinct. 



In general the pulsative images of red, or of red and orange mixed, 

 are of a blue-green tint, exceeding in brightness and apparent satura- 

 tion those due to any other exciting colours. Perhaps the strongest 

 effect was observed when the colour-patch was illuminated by rays 

 from about A 6100 to X 6550, aperture No. 4 of the rotating 

 diaphragm being used for the white-light disc. No pulsative image 

 of the red can, however, be formed unless the luminosity of the patch 

 is fairly great. 



With the eyepiece methods a feeble pulsative image was excited by 

 red in the neighbourhood of the B line. Its hue appeared bluish with 

 a slight tinge of green. In other respects the results for red were 

 similar to those obtained by Method I. 



Orange. — A colour-patch was formed by mixing rays from X 5800 to 

 X 6150. Its ordinary after-image was bright sky-blue. The pulsa- 

 tive image upon the screen appeared a rather dull blue-green with 

 aperture No. 2 of the rotating diaphragm and green-blue with 

 apertures 3 and 4. The eyepiece method showed the colour as 

 blue-green, paler than that excited by red. 



Yellow. — The ordinary after-image of a patch of yellow, X 5700 to 

 X 5890, was blue-violet. The tint of the pulsative image on the screen 

 was a pale nearly neutral grey, pinkish when the illumination was 

 weak, bluish when it was strong. A slightly more orange yellow, 

 X 5700 to X 5980, gave an image of nearly the same character but 

 a little stronger. When the eyepiece methods II and III were 

 employed with yellow, the pulsative images were exceedingly feeble, 

 and generally appeared to contain a trace of pink. The image due 

 to a greenish-yellow, X 5590 to X 5740, was more decidedly pink or 

 pale purple. Similar effects were obtained when the exciting yellow 



