of the sea, the drifting of clouds, the fall of rain, the waving of trees in the wind, the rushing of water, the darting of meteors, the rising and setting of the sun-are all mixed impressions of spectacle and movement. In like manner, in the various processes of the arts, there are characteristic movements to constitute our means of discrimination, and our permanent notions of those processes. The evolutions of an army have to be remembered as movements, and therefore need to be embodied among the muscular recollections of the system.

14. Visible Form.-We have taken the case of moving objects as the least complicated experience of vision. We must now enquire by what process we perceive Visible Form and Extension, and acquire the notion of Simultaneous existence in Space. It has to be shown that the eye is active in the observation even of still life; the special mode of activity being such as to make the mind feel the difference between Succession and Co-existence.

When we follow a moving object, as a rocket, or a bird, and when we carry the eye along the curve of the rainbow, there is a common fact of movement, with important differences in the mode. These differences are, to a great degree, parallel to those described under Touch, whereby the knowledge of objects as co-existing is attained. (1.) In the first place, in following the outline of the rainbow, we are not constrained to any one pace of movement, as with a bird or a projectile. This alone would give a lively sense of difference between the two appearances. (2.) In the next place, the optical impression, in the case of a still form, is not one unchanging sensation, but a series of sensations, which may be of the same natureas in the rainbow, or may be all different-as in sweeping across the clouds or the landscape. (3.) Thirdly, we can, by an inverted movement, encounter the same series of optical sensations, in the inverted order; whereas, in the other case, the object passes finally away from the sight. (4.) In the fourth place, we may repeat the movement, at any rate of speed, and in so doing obtain the same series of sensations, in the same

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order. Both in touch and in sight, this circumstance is probably what, more than anything else, gives us that vivid sense of the difference between objects moving and passing away-thereby typifying Succession-and objects that are simultaneous or co-exist, which is the meaning of Space. The more frequently we experience this fixed recurrence of optical sensations, in company with a definite movement, the broader is the line between that mode of existence and the objects that give us only one chance of observing them. The constant reception of a definite series of sensations by one definite movement, and the equally constant occurrence of the series inverted under an inverted movement, go far to make up our notion, meaning, and expectation, of objects extended in Visible Space.

But, (5.) in the fifth place, as regards Sight, the peculiar power of the eye to embrace at one glance a wide prospect, although minutely perceiving only a small portion, is available to confirm the same distinction. When the glance is carried along the field of view, the portions that cease to occupy the centre of the eye, still impress the retina, and have a place in the consciousness, though much less distinctly perceived. This constitutes an additional distinction between the transitory flight of a meteor and the picture of the starry sphere. Touch possesses this means of discrimination only in a very limited degree. The extended surface of the hand, the plurality of fingers, the united touch of the two hands, and such extent of the surface of the body as can make a simultaneous contact,-are all that there is to correspond to this great prerogative of sight, in giving a plurality of simultaneous impressions, so as to mark the difference between the co-existing in Space and the successive in Time. When a definite series of successive sensations are simultaneously felt, they suggest all the separate facts of movement, together with the whole fact of movement, involved in a perception of the Extended.

Thus, then, the observation of the forms of still life is a combination of the movements of the eye, with the optical

impressions corresponding to the different parts of the field of view. Exactly as in the case of moving things-by a horizontal sweep, we take in a horizontal line; by a circular sweep, we derive the muscular impression of a circle; by a sudden change of direction, we are cognizant of an angle; there being, in all these instances, the persistence on the retina of the whole figure, while the eye scans the successive parts.

The transition is easy from Lines to Surfaces. A more numerous and complex series of movements is requisite to give the impression of a visible area or superficies. But the same constant series of optical effects, imbedded in the same movements, inverted and repeated as oft as we please, enters into the cognition of space in two dimensions, as well as into the perception of linear magnitude, or space viewed in one dimension.

15. Apparent Size.-The apparent size or visible magnitude is made up of the two discriminations-optical and muscular. The Optical discrimination takes place through the extent of the image on the retina; hence the apparent size is spoken of by Wheatstone as the retinal magnitude. The Muscular discrimination depends upon the sweep of the eye under the action of its muscles; and is, therefore, a fact or experience of our muscular energy or activity. The two estimates co-operate to a joint result. They are both equivalent to an angular estimate, or the proportion of the visible surface to a whole sphere. The apparent diameter of the sun or moon is half a degree, or of the circle of the sky.

The combined estimate of Retinal Magnitude, by our two most sensitive organs-the retina and the ocular group of muscles-renders our measurement of apparent size singularly delicate. In fact, this is the finest discrimination within the compass of our senses; and whenever we desire to measure any property with nicety, we endeavour to resolve the case into a comparison of visible magnitudes. Of this description are the standards of weight (the balance), of heat (the thermometer), and many others.

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The fluctuations of visible magnitude in consequence of changes of Distance are appreciated with similar delicacy; and after we are aware that these fluctuations correspond to alteration of real distance, we use them as the most delicate test of remoteness.

The celestial bodies and the clouds are conceived by us solely under their apparent or visible size. Terrestrial objects, being seen by us at different distances, vary in apparent size, and we conceive most of them under a more or or less perfect estimate of their real size, as ascertained by handling and locomotion. Failing this estimate, we adopt some one point of view, which we have been most accustomed to, and conceive the object, as seen from thence. In regard to very familiar things, as a chair, or a man, we uniformly translate the apparent estimate into a real estimate. A building, a distant mountain, a landscape, are visually conceived as they appear from our most usual position with reference to them.

16. Distance, or varying remoteness.-The apparent size, as above considered, includes only two dimensions. In order to appreciate apparent volume or solidity, as an advance upon mere extension, or surface, we must estimate varying remoteness also.

Leaving out, at the present stage, the consideration of real distance, as well as real magnitude, we may advert to the various ocular sensibilities affected by alteration of distance.

We have already remarked on the two muscular adaptations of the eye to distance,-the change in the eye-ball by the operation of the ciliary muscle, for near distances, and the convergence or divergence of the two eyes, for distances both near and far. To preserve a distinct image when an object is brought nearer, we need, by a muscular effort, to change the curvature of the crystalline lens in each eye, and to make the lines of sight of the two eyes converge. Both these efforts are attended with consciousness, and this consciousness mingles with the feelings of altered retinal mag

nitude, and with dissimilarity of binocular images, when objects retreat from the eye, or advance toward it; while, in addition, the optical fact of varying clearness may also tell, together with the presence or absence of intervening objects.

17. Visible Movements and Visible Forms in three dimensions; Volume.-By combining the visible movements across the field of view with the movements of adjustment-monocular and binocular-we attain the experience of visible movements, visible forms, and visible magnitudes, in all the three dimensions of space; in other words, volume and solidity, in so far as these are understood by the eye alone. An object moving aslant requires changes of adjustment along with the movements of the eyeball, right or left, up or down; and its image remains embodied in this more complicated series of movements and optical changes. A row of houses seen obliquely, needs the same combination. With the lateral movements of the eye, we must unite adjusting movements, in order to maintain the same distinctness of picture throughout. These changes of adjustment are repeated and inverted, along with the other movements, and conspire with these to give the sense of the co-existing in space, as opposed to the passing or successive in time.

18. The intellectual imagery derived through the eye from the forms of still life is co-extensive with the visible creation. For the purposes of discriminating and of identifying natural things, and also for the storing of the mind with knowledge and thought, the sensations of objects of sight are available beyond any other class. The eye is kept constantly at work upon the surrounding scene, following the outlines and windings of form, as these extend in every direction; and, by the movements thus stimulated, each sparate object is distinguished from those that differ in shape, size, or distance, and identified with itself and those that coincide with it in these peculiarities. The train of movements for a square are recognized as distinct from the train that describes an oval: the outline of a pillar brings on a cycle of motions wholly