their Relation to certain oilier Visual Phenomena. 271 



became easier to judge of the colour of the image. The iris-diaphragm 

 was covered with ground-glass. 



Method IV. — This is not a colour-patch method, but an ordinary 

 spectroscopic one, the unmixed spectrum as dispersed by the prism 

 being viewed through a tubeless telescope. The eyepiece V (fig. 7) 



Fig. 6. Fig. 7. 



occupies the place of the slit-screen behind the disc H ; white light is 

 reflected into the eyepiece by the silvered mirror S and the clear 

 plate-glass T, as in Method II. A sheet of ground-glass takes the 

 place of the iris-diaphragm, which is removed. The arrangement is 

 in all essential respects similar to that adopted by Mr. Burch,* except 

 that the reflected white light is derived from the electric arc instead 

 of from the sky, its intensity being capable of wide variation. About 

 one-third of the whole length of the spectrum can be seen at once ; the 

 eyepiece is so directed that the spectrum may occupy only the lower 

 half of the field, while the white light, when admitted, fills the whole 

 of it. 



Ordinary Negative After-images. — The apparatus, whether arranged 

 for the projection of a colour-patch upon a screen or for observation 

 with an eyepiece, is exceedingly well adapted for the study of ordinary 

 negative after-images. The zinc disc H is set so that a coloured 

 image is formed upon a black ground ; after this has been gazed at 

 for 10 or 20 seconds, it is obliterated by turning the disc through a 

 * ' Boy. Soc. Proc.,' vol. 66, p. 215. 



