36 Mr. G. J. Burch. 



General Appearance of the Spectrum during Temporary Colour 

 Blindness. 



A large spectroscope, in which only about a tenth part of the 

 spectrum is visible at once, is directed to the sun, and the slit opened 

 till the illumination is as intense as can be borne. After the eye has 

 been sufficiently fatigued the results are observed in a single-prism 

 spectroscope in which the entire speotrum is visible. 



The following parts of the spectrum, namely, the red from A to B, 

 the green from the neighbourhood of E, the blue about half-way 

 between F and Gr, and the violet at and beyond H, produce well- 

 defined and characteristic results, whereas the intermediate portions 

 of the spectrum produce results intermediate in character. 



That is to say, while exposure to red light causes changes affecting 

 the red, and exposure to green light produces corresponding changes 

 in the green, yellow light, instead of causing corresponding changes 

 in the yellow, affects the whole of the red and the whole of the 

 green, the total change being equal to the sum of the changes due to 

 excitation by red light and by green light separately. 



The effects produced by each of the four above-mentioned colours 

 differ in degree but not in kind. 



(1) In each case all direct sensation of the colour used for 

 fatiguing the eye is lost. 



(2) There is produced a positive after-effect of the same colour by 

 which the hue of all other colours is modified if they are relatively 

 weak, but which is unnoticed if they are bright. 



(3) The temporary abolition of any one colour sensation is with- 

 out effect on the intensity of the remaining colour sensations. 



(4) Any two or any three of these four colour sensations can be 

 simultaneously or successively exhausted. 



(5) The positive after-effect of red is very transient ; that of 

 green lasts longer and is more noticeable ; that of blue is still more 

 powerful and persistent ; and that of violet is strongest and lasts a 

 long while. As the positive after-effect subsides the colour sensa- 

 tion returns, but the positive after-effect becomes unnoticeable long 

 before the colour sensation is restored to its full strength. 



(6) During the process of dazzling the eye the observer is con- 

 scious of the progress of the change, but only realises the extent of 

 his colour blindness on attempting to examine a less brilliantly 

 illuminated spectrum. 



The positive after-effect does not in these experiments pass through 

 cyclic changes of tint as after-images appear to do under other con- 

 ditions. 



