their Relation to certain other Visual Phenomena. 263 



having an open sector, as shown in fig. 1. If such a disc is caused 

 to turn five or six times in a second, while its surface is strongly 

 illuminated, a coloured object placed behind it and viewed inter- 

 mittently through the open sector, generally appears to assume an 

 entirely different hue, which is approximately complementary to the 

 true colour of the object : a piece of red ribbon, for example, is seen 

 as greenish-blue and a green one as pink. 



Fig. 1. Fig. 2. 



The tints thus produced are referred to in the paper as " pale " 

 ones. I have since found that their intensity may in most cases be 

 greatly increased if the object is illuminated more strongly than the 

 disc. The best arrangement for the purpose is indicated in plan in 

 fig. 2, where is the coloured object, e.g., a design painted on a card, 

 L, L are two incandescent electric lamps of fifty candle-power, and 

 K is a third lamp of thirty-two candle-power, supported horizontally a 

 little above the axis of the disc : all three lamps are fitted with metal 

 hoods to screen the light from the observer's eyes. The distance of 

 the lamp K from the disc may be varied until the best results are 

 obtained. When only a single lamp is used for illuminating both the 

 object and the disc (as in the original arrangement), the light portion 

 of the disc should be covered with paper of a pale neutral tint (not 

 bluish), reflecting about half as much light as ordinary white paper ; 

 for experiments in bright diffused daylight, the paper may advan- 

 tageously be of a pale yellowish-grey or buff tint. The dark part of 

 the disc should be covered with good black velvet, and the open sector 

 should extend to about 70°, instead of only 45°, as recommended in 

 the former paper. 



A number of observations made from time to time with the appara- 

 tus as thus modified have shown that the " pulsative " after-images, as 

 they will be called, differ in several important respects from the 

 " ordinary " negative after-images seen upon a white or grey back- 

 ground after the gaze has been fixed for some seconds upon a coloured 

 object. The colours of the pulsative after-images produced by certain 

 hues of red and of green may appear far more intense or saturated 

 than those of the ordinary negative after-images excited by the same 



