han that supposition of a Medium of which w speaking. Thus Bacon, after reviewing th omena of sound, says', 'Videntur motus soni fie www.spirituales: ita enim loquendum donec ce appiam inveniatur.'

though the fundamental principles of several scienc Jepoad upon the assumption of a Medium of Perceptio De principles do not at all depend upon any speci wow of the Process of our perceptions. The mechanis chat process is a curious subject of consideration

belongs to physiology, more properly than eith metaphysics, or to those branches of physics of whi we are now speaking. The general nature of the pr is the same for all the senses. The object affec ão appropriate intermedium; the medium, throu the proper organ, the eye, the ear, the nose, affects t HOO of the particular sense; and, by these, in sor w, the sensation is conveyed to the mind. But

at the impression upon the nerves as the act of se tion which we have to consider, would be to mista our object, which is not the constitution of the hum body, but of the human mind. It would be to mista

link of the chain for the power which holds t oud of the chain. No anatomical analysis of the c pocal conditions of vision, or hearing, or feeling war is necessary to the sciences of Optics, or Acoustics, Chormotics.

Not only is this physiological research an extrane part of our subject, but a partial pursuit of such a search may mislead the inquirer. We perceive obje by means of certain media, and by means of cert pressions on the nerves: but we cannot with ioty say that we perceive either the media or impressions on the nerves. What person in the act sooing is conscious of the little coloured spaces on retina? or of the motions of the bones of the audit apparatus whilst he is hearing? Surely, no one. may appear obvious enough, and yet a writer of common acuteness, Dr. Brown, has put forth seve

7 Hist. Son. et Aud. vol. ix. p. 87.

T

very strange opinions, all resting upon the doctrine that the coloured spaces on the retina are the objects which we perceive; and there are some supposed difficulties and paradoxes on the same subject which have become quite celebrated (as upright vision with inverted images), arising from the same confusion of thought.

As the consideration of the difficulties which have arisen respecting the Philosophy of Perception may serve still further to illustrate the principles on which we necessarily reason respecting the secondary qualities of bodies, I shall here devote a few pages to that subject.

CHAPTER II.

ON PECULIARITIES IN THE PERCEPTIONS OF THE

DIFFERENT SENSES.

I.

WE

E cannot doubt that we perceive all secondary qualities by means of immediate impressions made, through the proper medium of sensation, upon our organs. Hence all the senses are sometimes vaguely spoken of as modifications of the sense of feeling. It will, however, be seen, on reflection, that this mode of speaking identifies in words things which in our conceptions have nothing in common. No impression on the organs of touch can be conceived as having any resemblance to colour or smell. No effort, no ingenuity, can enable us to describe the impressions of one sense in terms borrowed from another.

The senses have, however, each its peculiar powers, and these powers may be in some respects compared, so as to show their leading resemblances and differences, and the characteristic privileges and laws of each. This is what we shall do as briefly as possible.

SECT. I.-Prerogatives of Sight.

THE sight distinguishes colours, as the hearing distinguishes tones; the sight estimates degrees of brightness, the ear, degrees of loudness; but with several resemblances, there are most remarkable differences between these two senses.

2. Position. The sight has this peculiar prerogative, that it apprehends the place of its objects directly and primarily. We see where an object is at the same instant that we see what it is. If we see two objects, we see their relative position. We cannot help per

ceiving that one is above or below, to the right or to the left of the other, if we perceive them at all.

There is nothing corresponding to this in sound. When we hear a noise, we do not necessarily assign a place to it. It may easily happen that we cannot tell from which side a thunder-clap comes. And though we often can judge in what direction a voice is heard, this is a matter of secondary impression, and of inference from concomitant circumstances, not a primary fact of sensation. The judgments which we form concerning the position of sounding bodies are obtained by the conscious or unconscious comparison of the impressions made on the two ears, and on the bones of the head in general; they are not inseparable conditions of hearing. We may hear sounds, and be uncertain whether they are 'above, around, or underneath!' but the moment anything visible appears, however unexpected, we can say, 'see where it comes!'

Since we can see the relative position of things, we can see figure, which is but the relative position of the different parts of the boundary of the object. And thus the whole visible world exhibits to us a scene of various shapes, coloured and shaded according to their form and position, but each having relations of position to all the rest; and altogether, entirely filling up the whole range which the eye can command.

3. Distance.-The distance of objects from us is no matter of immediate perception, but is a judgment and inference formed from our sensations, in something of the same way as our judgment of position by the ear, though more precise. That this is so, was most distinctly shown by Berkeley, in his New Theory of Vision. The elements on which we form our judgment are, the effort by which we fix both eyes on the same object, the effort by which we adjust each eye to distinct vision, and the known forms, colours, and parts of objects, as compared with their appearance. The right interpretation of the information which these circumstances give us respecting the true distances and forms of things, is gradually learnt by experience, the lesson being begun in our earliest infancy, and inculcated upon us every hour during which we

use our eyes. The completeness with which the lesson is learnt is truly admirable; for we forget that our conclusion is obtained indirectly, and mistake a judgment on evidence for an intuitive perception. This, however, is not more surprizing than the rapidity and unconsciousness of effort with which we understand the meaning of the speech that we hear, or the book that we read. In both cases, the habit of interpretation is become as familiar as the act of perception. And this is the case with regard to vision. We see the breadth of the street as clearly and readily as we see the house on the other side of it. We see the house to be square, however obliquely it be presented to us. Indeed the difficulty is, to recover the consciousness of our real and original sensations; discover what is the apparent relation of the lines which appear before us. As we have already said,

to

(book ii. chap. 6) in the common process of vision we suppose ourselves to see that which cannot be seen; and when we would make a picture of an object, the difficulty is to represent what is visible and no

more.

But perfect as is our habit of interpreting what we perceive, we could not interpret if we did not perceive. If the eye did not apprehend visible position, it could not infer actual position, which is collected from visible position as a consequence: if we did not see apparent figure, we could not arrive at any opinion concerning real form. The perception of place, which is the prerogative of the eye, is the basis of all its other superiority.

The precision with which the eye can judge of apparent position is remarkable. If we had before us two stars distant from each other by one-twentieth of the moon's diameter, we could easily decide the apparent direction of the one from the other, as above or below, to the right or left. Yet eight millions of stars might be placed in the visible hemisphere of the sky at such distances from each other; and thus the eye would recognize the relative position in a portion of its range not greater than one eight-millionth of the whole. Such is the accuracy of the sense of vision in this