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, observing, is a double process, partly objective or outward--the thing seen and the retina--and partly subjective or inward--the picture mysteriously transferred to the mind's representative, the brain, and there received and affiliated with other images. Illustrations of such seeing "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 curiosities 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 persons 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 observer are all these views, though visible, really seen. [Illustration: FIG. 1.[5]--The man's face in the rocks is quite distinct, and is usually readily found when it is known that there is a face somewhere. (For this view from the Dalles of the St. Croix, Minn., I am indebted to the courtesy of Mr. W. H. Dudley, of Madison, Wis.)] 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 quite irresistible. We turn it about in all directions, wondering where the hidden form can be, scanning every detail of the picture, until suddenly a chance glimpse reveals it, plainly staring us in the face. When several persons are engaged in this occupation, it is amusing to observe how blind each is to what the others see; their physical eyes see alike, but their mental eyes reflect their own individualities. [Illustration: FIG. 2.--In order to see the lion's head, hold the dollar exactly inverted and the head will be discovered facing the left, as above outlined. It is clearer on the dollar itself than in this reproduction.]