This is the reason why the sun, moon, planets, and fixed stars appear to be all at the same distance, as if they touched the concave surface of a great sphere. The sphere itself is at that distance beyond which all objects affect the eye in the same manner.

Another mode in which we determine the distance of objects is by referring them to those intervening objects whose distance is known. We are so much accustomed to measure with our eye the ground which we travel, and to compare the judgments of distance formed by sight with our experience or information, that we learn by degrees in this manner to form a more accurate judgment of the distance of terrestrial objects than we could do by any of the means above mentioned. It is for want of some intervening objects that it is so difficult to measure distances by the eye up in the air, out at sea, or on extensive plains. This mode of estimating distance, accounts for the superior apparent magnitude of the moon in the horizon: for, first, its distance seems greater on account of the known distance of the terrestrial objects that intervene; and where the visible magnitude is the same, the real magnitude of objects is always determined to be in proportion to the distance.

The proof of this being the real solution of the difficulty is, that if the horizontal moon be viewed through a tube which excludes all terrestrial objects, its appearance is precisely the same as at any other time.

The last method by which we determine the distance of objects is by their visible magnitude. By experience, I know what figure a man or any other known object makes to my eye at the distance of ten feet; I perceive the gradual diminution of this visible figure at the distance of twenty, forty, one hundred feet, till it vanish altogether: hence a certain visible magnitude of a known object becomes the sign of a certain determinate distance, and carries along with it the conception and belief of that distance.

I shall say nothing here of the moral method of measuring distances; - the distance from home to school, in the days of our youth, being generally double the distance from school to home; and so forth with all other passions which quicken or retard the feeling of time.

It is just the same with the cubical magnitudes of bodies. We think we see that a body is thick and round; it is quite certain that we see neither the one nor the other, for the eye can see nothing but plain surfaces; but then we learn from experience that certain different appearances of light or shade upon plain surfaces are constantly connected with those feelings of bodies which we call round and thick. Just in the same manner it is probable that the notions which the ear has of distance and position are entirely the result of experience; and that a person deaf from his birth, and suddenly cured, would be quite ignorant from what quarter, and from what distance, sound originated. Thus we see that the senses soon learn to lay aside their own homely and barren language, and to speak in a more elegant and universal dialect; and we see that man, endowed with the senses he now is, and deprived of the power of connecting their notices together by indissoluble associations, would have risen very little above the rank of the lower animals. All the labours of the human mind point and tend towards the same process which has been carried on in our early infancy with respect to associated sensation, so to connect together, by copious induction, the sign with the thing signified, that the one may suggest the other with the certainty and velocity of

sensation.

The phenomena of double vision and inverted images I must, for fear of protracting my lecture too long, entirely pass over; referring those whose curiosity may be excited on these subjects to Bishop Berkeley's Essay

on Vision, Dr. Porterfield on the Eye, Dr. Wells' Essay on Vision, and Dr. Reid's admirable first work on the Human Mind. To prove, in some measure, how much of our sight is original, and how much acquired, and to illustrate therefore a great deal of what I have said throughout this lecture, I shall read to you the famous case of a young man born blind, and suddenly restored to his sight by undergoing the operation of couching.

A young gentleman, who was born with two cataracts upon each of his eyes, was, in 1728, couched by Mr. Cheselden, and by that means for the first time. made to see distinctly. "At first," says the operator, "he could bear but very little light, and the things he "saw he thought extremely large; but upon seeing things larger, those first seen he conceived less, never being able to imagine any lines beyond the bounds he "saw. The room he was in, he said, he knew to be but part of the house, yet he could not conceive that the "whole house would look bigger.

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"Though we say of this gentleman that he was blind, "as we do of all people who have ripe cataracts, yet "they are never so blind from that cause but that they " can discern day from night, and, for the most part, in "a strong light, distinguish black, white, and scarlet: "but they cannot perceive the shape of any thing; for "the light by which these perceptions are made, being "let in obliquely through the aqueous humour, or the "anterior surface of the crystalline humour, by which "the rays cannot be brought into a focus upon the "retina, they can discern in no other manner than a "sound eye can through a glass of broken jelly, where a great variety of surfaces so differently refract the light, that the several distinct pencils of rays cannot "be collected by the eye into their proper foci; where"fore the shape of an object in such a case cannot be

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"discerned at all, though the colour may: and thus it "was with this young gentleman, who, though he knew "those colours asunder, in a good light, yet, when he saw them after he was couched, the faint ideas he had "of them before, were not sufficient for him to know "them by afterwards; and therefore he did not think "them the same which he had before known by those

"When he first saw, he was so far from making any "judgment about distances, that he thought all objects "whatever touched his eyes (as he expressed it), as "what he felt did his skin; and thought no objects so 66 agreeable as those which were smooth and regular, though he could form no judgment of their shape, or แ guess what it was in any object that was pleasing to "him. He knew not the shape of anything, nor any "one thing from another, however different in shape or "magnitude; but upon being told what things were "whose form he before knew from feeling, he would carefully observe, that he might know them again; "but having too many objects to learn at once, he forgot many of them, and (as he said) at first learned to know, "and again forget, a thousand things in a day. One particular only, though it may appear trifling, I will "relate. Having often forgot which was the cat and "which the dog, he was ashamed to ask; but catching "the cat (which he knew by feeling), he was observed "to look at her steadfastly, and then, setting her down, "said, 'So, Puss! I shall know you another time.'

"We thought he soon knew what pictures represented "which were shown him; but we found afterwards we แ were mistaken, for, about two months after he was "couched, he discovered at once they represented solid bodies, when to that time he considered them only as "party-coloured planes, or surfaces diversified with variety of paints: but even then, he was no less sur

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"prised, expecting the pictures would feel like the "things they represented; and was amazed when he "found those parts which, by their light and shadow, "appeared now round and uneven, felt only flat like the rest, and asked which was the lying sense, feeling "or seeing.

"In a year after seeing, the young gentleman being "carried upon Epsom Downs, and observing a large prospect, he was exceedingly delighted with it, and "called it a new kind of seeing."