1 894] 



Images following Visual Impressions. 

 Fia. 3. 



143 



(4) After the external light has been shut off, a sensation of 

 diminishing luminosity continues for a short time, and is suc- 

 ceeded by a brief interval of darkness. 



(5) Then follows a sudden and clearly-defined sensation of what 

 may be called abnormal darkness darker than common dack- 

 ness which lasts for about one-sixtieth of a second, and is 

 followed by another interval of ordinary darkness. 



(6) Finally, in about a fifth of a second after the extinction of the 

 external light, there occurs another transient impression of 

 luminosity, generally violet coloured, after which the uni- 

 formity of the darkness remains undisturbed. 



An attempt has been made in fig. 3 to give a rough diagrammatic 

 representation of the above-described chain of sensations. No 

 account has been taken of the comparatively feeble after-image, to 

 which the phosphorescent trail before referred to is due, and which 

 may last for two seconds or more. 



In conclusion, it may not be unnecessary to add a warning that, 

 though all the effects here described have been witnessed without 

 much difficulty by several persons besides myself, it is hardly probable 

 that any one, who is quite unaccustomed to observations of the kind, 

 will be entirely successful in a first attempt at repeating the ex- 

 periments. 



Addendum. May 24th. 



Since the above was written, there has been brought to my notice 

 an important paper by Dr. Carl Hess, " On the After-images follow- 

 ing luminous Impressions of short Duration."* In his principal 

 experiments momentary illumina'tion was produced by means of an 

 instantaneous shutter, giving an exposure of 1/100 or 1/200 second. 

 * Pfluger's ' Archly fur Physiologic,' vol. 49 (1891), p. 190. 



