3 oo POPULAR SCIENCE MONTHLY. 



eyelid cap, its iris shutter, its lens, and its sensitive plate — the retina ; 

 when properly adjusted for distance and light, the image is formed 

 on the retina as on the glass plate, and the picture is taken. So far 

 the comparison is helpful; but while the camera takes a picture 

 whenever and wherever the plate happens to be exposed, the com- 

 plete act of seeing requires some co-operation on the part of the mind. 

 The retina may be exposed a thousand times and take but few pic- 

 tures; or perhaps it is better to say that the pictures may be taken, 

 but remain undeveloped and evanescent. The pictures that are de- 

 veloped are stacked up, like the negatives in the photographer's shop, 

 in the pigeonholes of our mental storerooms — some faded and 

 blurred, some poorly arranged or mislaid, some often referred to 

 and fresh prints made therefrom, and some quite neglected. 



In order to see, it is at once necessary that the retina be suitably 

 exposed toward the object to be seen, and that the mind be favorably 

 disposed to the assimilation of the impression. True seeing, observ- 

 ing, is a double process, partly objective or outward — the thing seen 

 and the retina — and partly subjective or inward — the picture mys- 

 teriously transferred to the mind's representative, the brain, and there 

 received and affiliated with other images. Illustrations of such see- 

 ing " with the mind's eye " are not far to seek. Wherever the 

 beauties and conformations of natural scenery invite the eye of man 

 does he discover familiar forms and faces (Fig. 1); the forces of 

 Nature have rough-hewn the rocks, but the human eye detects and 

 often creates the resemblances. The stranger to whom such curiosi- 

 ties of form are first pointed out often finds it difficult to discover 

 the resemblance, but once seen the face or form obtrudes itself in 

 every view and seems the most conspicuous feature in the outlook. 

 The flickering fire furnishes a fine background for the activity of the 

 mind's eye, and against this it projects the forms and fancies which 

 the leaping flames and the burning embers from time to time suggest. 

 Not all see these fire-pictures readily, for our mental eyes differ more 

 from one another than the physical ones, and perhaps no two per- 

 sons see the same picture in quite the same way. It is not quite true, 

 however, as many have held, that in waking hours we all have a 

 world in common, but in dreams each has a world of his own, for 

 our waking worlds are made different by the differences in what 

 engages our interest and our attention. It is true that our eyes when 

 open are opened very largely to the same views, but by no one ob- 

 server are all these views, though visible, really seen. 



This characteristic of human vision often serves as a source of 

 amusement. The puzzle picture with its tantalizing face, or animal, 

 or what not, hidden in the trees, or fantastically constructed out of 

 heterogeneous elements that make up the composition, is to many 



