144 Recurrent Images following Visual Impressions. [June 7, 



Observations were made of the effects following excitation by white 

 light, by coloured light derived from different portions of the spec- 

 trum, and by the whole spectrum at once. In all his experiments, 

 Dr. Hess noticed the occurrence of what he speaks of as a negative 

 after-image of very short duration (corresponding to what I have 

 called an interval of darkness) which followed almost immediately 

 upon the termination of the illumination, and preceded what is 

 " commonly known as the positive after-image." He states that this 

 negative after-image which, according to his measurements, lasted 

 for about one-third or one-half second, was overlooked by Helmholtz, 

 Aubert, Fich and others. Hering, however, had reasons for suspect- 

 ing its existence, and it was to test this point that the experiments 

 in which Hering himself co-operated, were undertaken. 



The negative after-images are stated not to have been represented 

 in all cases by mere darkness. Under favourable conditions, the 

 " dark after-image " succeeding a momentary excitation by coloured 

 light, was tinted with a colour complementary to the original one ; 

 and when the stimulus was generated by the complete spectrum, all 

 the complementary colours were seen for a short time after its disap- 

 pearance. No such complementary tints have ever revealed them- 

 selves in my own experiments, the space between the primai'y 

 luminous image and its ghost always appearing as simply dark. 



The colours assigned by Dr. Hess to the " positive after-images " 

 also differ from those observed by myself. In most cases he describes 

 the positive after-image as either having a feeble colour of the same 

 hue as that of the light employed for the stimulus, or as being 

 colourless. 



Dr. Hess considers, as I do, that the brightest portion of the posi- 

 tive after-image of the spectrum corresponds with the green, and 

 remarks that the brightness decreases gradually towards the more 

 refrangible end of the spectrum, and much more quickly towards 

 the less refrangible end. 



Such discrepancies as seem to exist between Dr. Hess's results and 

 my own may perhaps be accounted for by the very different methods 

 of observation which we employed. A stationary stimulus would, no 

 doubt, be better adapted than a moving one for developing the feeble 

 tints of the dark negative after-images, as well as those exhibited by 

 the bright positive after-images during by far the greater part of 

 their continuance, which, according to Dr. Hess's estimate, is as long 

 as from four to eight seconds. On the other hand, the method 

 adopted by myself discloses the important fact, of which Dr. Hess 

 makes no mention whatever, that the positive images are immensely 

 brighter for a very brief initial period not more than one-tenth of a 

 second than during their subsequent existence. While this phase 

 of transient brilliancy altogether failed to attract Dr. Hess's notice, 



