be recalled after it has given place to another by pressure on the eyeball, either by blinking the eyes, or by touching with the hand.
The image also seems to wax and wane. This is probably due to the pulsation of blood in the arteries, for I have noticed that by moving my head I could change to some extent the colours of the image. When the eyes are shut and exposed to the light the colours of the image are, as a rule, brighter than when the eyelids are covered.
By observing the after-image my attention has frequently been called to some detail which I had not noticed in looking at the concrete object.
I will now give the results of a few experiments I have made for the purpose of noting the change of colour in after-images.
(1.) I was in a room with the window nearly covered with a dark-green blind, which did not quite reach to the top. The sun was shining brightly through this exposed part. Avoiding the glare, I looked for a second at the blue sky, which through the glass appeared almost white, then shut my eyes. Immediately I got a beautiful image—bright blue, with a dark stripe marking the wooden frame dividing the panes (which, by the way, I had not noticed with my eyes open). The blue changed into a bright violet, with a dark stripe, then to a rich yellow, which turned quickly into a deep brown with a light-grey or steel-coloured stripe down the centre. This negative image faded gradually into the surrounding dark background.
This experiment I repeated several times, with the same general results, but I found that every change in my position, by facing the light or turning my back to it, produced a different sensation.
So persistent was this after-image that I could open my eyes, look around the room without seeing the image, and shut my eyes again, when the image immediately reappeared.
(2.) I looked at the sun shining in the blue sky. The changes in colour were so rapid that I could with difficulty take note of them as they passed. But the general results were as follows: A white spot with a bright purple halo appeared in a darker purple background. The halo quickly disappeared, and the white spot changed to blue, still with a purple background; then the purple gave place to yellow, turning to orange, while the spot became yellow with a reddish background. I here pressed my eyeballs, with the result that the blue returned for an instant, but was followed immediately by orange. For a short time a green tint covered both spot and background, and after this had passed the red
