280 Dr. S. Biclwell. On Negative- After-images, and 



which will obstruct the red rays ; possible sources of error due to the 

 reflection of red light by the prism are thus avoided. The origin of 

 these blue-green borders is, no doubt, analogous to that of the red 

 borders, but the matter requires more careful and thorough investiga- 

 tion than it has yet received. 



Though the image of the needle was colourless when the patch was 

 illuminated by the greenish-yellow rays of the spectrum, it appeared 

 red when the same hue was formed by combining red and green rays. 

 Eed borders were also observed with a purple composed of red and 

 blue rays, with a white composed of red, green, and violet rays, and 

 with another white formed by recombining the whole of the spectrum ; 

 this last observation was, of course, practically a mere repetition in a 

 slightly different form of the one which formed the chief subject of my 

 previous paper. 



No coloured border of the same class has yet been observed when 

 the colour-patch was illuminated by the violet rays of the spectrum, 

 Method II being the one employed. The edge of the yellow pulsative 

 image was fringed with a pale violet rim, which, however, was wholly 

 inside the geometrical boundary of the image and not external to it, as 

 were the red and the blue-green borders. Red was very carefully 

 looked for around the violet, but not found. The so-called " simul- 

 taneous contrast " effect was, however, very remarkable, the whole field 

 of the eyepiece appearing of a strong yellow tint ; often it was quite 

 as strong as the colour of the image itself, which could only be dis- 

 tinguished from the background by the narrow violet ring surrounding 

 it. An equally remarkable effect was produced when the stimulating 

 light was blue, the " contrast-colour " being, like that of the image, 

 orange. 



VIII. Discussion of the Observations. 



Nature of the Pulsative Image. — The phenomenon which, for brevity, 

 has been termed the " pulsative after-image," may be denned as the 

 negative after-image of a coloured object which is seen against a white 

 ground after a very brief stimulation — 1/60 to 1/30 of a second — 

 following a period of repose. A strange peculiarity incidental to the 

 formation of these after-images is, that under suitable conditions of 

 illumination, the true colour of the light to which the phenomenon is due 

 altogether fails to evoke its appropriate sensation and is not perceived 

 at all, the only colour seen being that of the after-image. The diffi- 

 culty experienced in attempts to find a really definite explanation of 

 this fact, and illustrate it by curves of sensation, is in some degree 

 diminished by the singular observations upon the " black spot." The 

 black spot is, of course, merely a device for exhibiting a certain 

 border effect in a convenient manner. A small disc of green light is 



