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. DISTANCE FROM THE EYE. 237 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. building, a distant mountain, a landscape, are visually conceived as they appear from our most usual position with reference to them. A 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 INTELLECTUAL COMPASS OF THE EYE. 239 different from those dictated by the figure of a tree. The property belonging to the mental system of causing to cohere movements that have been described in succession, fixes the series for each different view, and gives a permanent hold of all the distinct forms presented to the eye. This adhesive process belongs to the intellect, and will be fully treated of in the proper place. CHAPTER III. OF THE APPETITES. 1. IN taking up, at this stage, the consideration of the Appetites, I do not mean to assert that these entirely belong to our primitive impulses, or that in them the operation of intellect and experience is excluded. On the contrary, I am of opinion that Appetite, being a species or form of Volition, is like all our other effective forms of volition, a combination of instinct and education. But the process of acquisition is in this case simple and short; while, on the other hand, the stimulus to action, or the source of the craving, is usually one of the sensations or feelings discussed in the two previous chapters. Indeed, if we look at the craving alone, without reference to the action for appeasing it, that craving is merely what we have all along styled the volitional property of the sensation. If a spur to action were to constitute Appetite, all our pains and pleasures would come under this designation. But the Appetites commonly considered are a select class of feelings; and are circumscribed by the following propertynamely, that they are the cravings produced by the recurring wants and necessities of our bodily, or organic life. The avoiding of a scald, a cut, or a fall, is an energetic impulse of volition, and yet not a case of appetite; there being no periodic or recurring want of the system in these cases. Sleep, Exercise, Repose, Thirst, Hunger, Sex, are the appetites most universally present throughout the Animal tribes. The state termed Desire so far agrees with Appetite, in being a volitional impulse growing out of some uneasy and unsatisfied condition. But in Desire, there is a prior experience of pleasure, the memory of which is the spur to seek a |