Browning. — On After-images. 507 



dark and the dark light. The bright spot seen after looking 

 at the setting sun is a good example of the first class ; and I 

 have obtained a very perfect negative image by looking at a 

 dark photograph in a white mount. After-images are sensa- 

 tions, the positive being due to the continuation of the ex- 

 citement of the nerve-centres, and the negative to the reaction 

 of the nerves after excitation. Later on T shall deal with the 

 changes of the positive into negative images. 



Fechner, who was one of the great authorities on this 

 subject, drew attention to many points of interest ; but, un- 

 fortunately, he lost his sight, and was unable to complete his 

 researches. It will be well to mention here that experiments 

 with after-images are very trying to the eyes, and should be 

 carried on with moderation. Many of Fechner' s observations 

 were taken in bed, and he describes how he found out that 

 memory and knowledge play a very important part in percep- 

 tion. Lying in bed with his eyes open, he was under the im- 

 pression that he could see the whole length of the bed, and in 

 the memory-image he perceived the bed as he believed he had 

 seen it. But the negative after-image showed clearly that the 

 bed was foreshortened in the retina. Again, in the after- 

 image the objects all appeared in one plane, while in the per- 

 ception one object seemed to be further away than another. 



Another peculiarity of after-images is that we cannot escape 

 from them. Most people have noticed how painfully persistent 

 is the after-percept of the sun or any bright light. This is due 

 to the fact that after-images are caused by the action of the 

 nerve-centres after excitement, and are not dependent on 

 external objects. To obtain a good after-image, look steadily 

 at a bright object, and then shut the eyes. The light will be 

 distinctly visible for some time — indeed, the degree of success 

 in obtaining a good image is largely dependent on the lumin- 

 osity of the object. I have been unable to keep an after- 

 image longer than a quarter of an hour. I was in London, 

 and the afternoon sun was shining brightly on a polished 

 shield, forming a brilliant point of light. I looked steadily at 

 this object, and then shut my eyes, and covered them so that 

 no light could interfere with the result. I then noted the 

 various changes, and remained with my eyes covered until I 

 could no longer perceive either a positive or negative image, 

 even by pressing my eyeball. I looked at my watch, and 

 found that nearly a quarter of an hour had elapsed. A lady 

 to whom I had been talking on this subject told me that she 

 had had a very remarkable revived after-image. She had been 

 picking violets during the day, and in the evening she saw dis- 

 tinctly the bank where she had found the flowers. 



The colours seen in these after-percepts are wonderfully 

 pure and bright, and it has been observed that one colour may 



