Page:Transactions NZ Institute Volume 25.djvu/653

 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 sensations, the positive being due to the continuation of the excitement of the nerve-centres, and the negative to the reaction of the nerves after excitation. Later on I 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, unfortunately, 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 perception. Lying in bed with his eyes open, he was under the impression 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 perception 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 luminosity 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 distinctly 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