Page:Popular Science Monthly Volume 40.djvu/641

Rh impressions made by the moving pageantry of the Arizona desert are curiously transferred to this crimson background. For I see a strip of plush moving irregularly to the right of me, and just above it another section moving to the left.

As the movements of the plush correspond very nearly to the previous visual impressions made by the moving landscape, I soon find that I can vary the plush movements at will.

Allowing sufficient intervals of rest to elapse, I am able to make an upper segment of the plush cushion move slowly backward or forward in contrast with a lower portion—a faithful photograph from the landscape negative.

This persistence of strong or continued retinal impressions may easily be demonstrated by another and commoner experiment. Look intently for two or three minutes at the light falling through a small window, other illumination being excluded. Then close the eyes and place a bandage over them. The impression produced by the light persists several minutes, and the experiment will be all the more striking if the window be crossed by bars, the persistent images of which are seen distinctly in strong contrast to the lighted spaces surrounding them.

Kühne, of Heidelberg, and others have shown that the retina possesses a pigmentary substance (visual purple), sensitive to light, which acts like the sensitized plate or film of the photographic camera, and that a picture distinctly seen is actually photographed upon the background of the eye.

Looking from the rear platform of our vestibule train—an admirable vantage-ground from which to view the country through which one is passing—I find that we have just skirted some foot-hills and are approaching the mouth of a small canon, at the head of which a bold, black mountain looks threateningly down on the desert below. The train once more gains the level country, and on looking back, although it is far up the gorge, the mountain seems very near. Nay, more, as I look first at the roadbed and then at the base of the huge mass in front of me, the latter, in some uncanny way, follows, as if it wished to fall upon and crush me. This apparent motion reminds me of Shelley's description:

 The Apennine in the light of day Is a mighty mountain dim and gray, Which between the earth and sky doth lay; But when night comes, a chaos dread On the dim starlight then is spread. And the Apennine walks abroad with the storm."

Not so, however, is it with this particular outpost of the Sierra Madre. The fact is that while I have, at the car window, been experiencing the retinal effects produced by objects moving in a