WAYSIDE OPTICS. 621 



impressions made by the moving pageantry of tlie 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 back- 

 ward or forward in contrast with a lower portion — a faithful pho- 

 tograph 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 pro- 

 duced 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. 



Kiihne, 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 photo- 

 graphic camera, and that a picture distinctly seen is actually pho- 

 tographed 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 road- 

 bed and then at the base of the huge mass in front of me, the lat- 

 ter, 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 

 Mad re. The fact is that while I have, at the car window, been 

 experiencing the retinal effects produced by objects moving in a 



