small body, such as a pea for example, be put between them, so as to touch the one on the right side and the other on the left, you would almost swear that you felt two peas. The explanation of this illusion depends on the same principles.

PROBLEM XII.

To cause an object, seen distinctly, and without the interposition of any opake or diaphonous body, to appear to the naked eye inverted.

CONSTRUCT a small machine, such as that represented fig. 17 pl. 4. It consists of two parallel ends, A B and CD, joined together by a third piece A C, half an inch in breadth, and an inch and a half in length. This may be easily done by means of a slip of card. In the middle of the end A B make a round hole E, about a line and a half in diameter; and in the centre of it fix the head of a pin, or the point of a needle, as seen in the figure: opposite to it in the other end make a hole F with a large pin; if you then apply your eye to E, turning the hole F towards the light or the flame of a, candle, you will see the head of the pin greatly magnified, and in an inverted position, as represented at G.

The reason of this inversion is, that the head of the pin being exceedingly near the pupil of the eye, the rays which proceed from the point F are greatly divergent, on account of the hole F; and instead of a distinct and inverted image, there is painted at the bottom of the eye a kind of shadow in an upright position. But inverted images on the retina convey to the mind the idea of upright objects: conse

quently, as this kind of image is upright, it must convey to the mind the idea of an inverted object.

PROBLEM XIII.

To cause an object, without the interposition of any body, to disappear from the naked eye, when turned towards it.

For this experiment we are indebted to Mariotte and though the consequences he deduced from it have not been adopted, it is no less singular, and seems to prove a particular fact in the animal

economy.

Fix, at the height of the eye, on a dark ground, a small round piece of white paper, and a little lower, at the distance of two feet to the right, fix up another, of about three inches in diameter; then place yourself opposite to the first piece of paper, and, having shut the left eye, retire backwards, keeping your eye still fixed on the first object when you have got to the distance of nine or ten feet, the second will entirely disappear from your sight.

This phenomenon is accounted for by observing, that when the eye has got to the above distance, the image of the second paper falls on the place where the optic nerve is inserted into the eye, and that according to every appearance this place of the retina does not possess the property of transmitting the impression of objects; for while the nervous fibres in the rest of the retina are struck directly on the side by the rays proceeding from the objects, they are struck here altogether obliquely, which destroys the shock of the particle of light.

PROBLEM XIV.

To cause an object to disappear to both eyes at once, though it may be seen by each of them separately.

AFFIX to a dark wall a round piece of paper, an inch or two in diameter, and a little lower, at the distance of two feet on each side, make two marks then place yourself directly opposite to the paper, and hold the end of your finger before your face in such a manner, that when the right eye is open, it shall conceal the mark on the left, and when the left eye is open the mark on the right; you then look with both eyes to the end of your finger, the paper, which is not at all concealed by it from either of your eyes, will nevertheless disappear.

if

This experiment is explained in the same manner as the former; for, by the means here employed, the image of the paper is made to fall on the insertion of the optic nerve of each eye, and hence the disappearance of the object from both.

PROBLEM XV.

An optical game, which proves that with one eye a person cannot judge well of the distance of an object.

PRESENT to any one a ring, or place it at some distance, and in such a manner that the plane of it shall be turned towards the person's face: then bid him shut one of his eyes, and try to push through it a crooked stick, of sufficient length to reach it: he will very seldom succeed,

The reason of this difficulty may be easily given: it depends on the habit we have acquired of judging of the distances of objects by means of both our eyes; but when we use only one, we judge of them very imperfectly

A person with one eye would not experience the same difficulty: being accustomed to make use of only one eye, he acquires the habit of judging of distances with great correctness.

PROBLEM XVI.

A person born blind, having recovered the use of his sight; if a globe and a cube which he has learnt to distinguish by the touch are presented to him, will he be able, on the first view without the aid of touching, to tell which is the cube, and which is the globe?

THIS is the famous problem of Mr. Molyneux, which he proposed to Locke, and which has much exercised the ingenuity of metaphysicians.

Both these celebrated men thought, not without reason, and it is the general opinion, that the blind man, on acquiring the use of his sight, would not be able to distinguish the cube from the globe, or at least without the aid of reasoning; and indeed, as Mr. Molyneux said, though this blind man has learned by experience in what manner the cube and the globe affect his sense of touching, he does not yet know how those objects which affect the touch will affect the sight; nor that the salient angle, which presses on his hand unequally when he feels the cube, ought to make the same impression on his eyes that it does on his sense of touching.

He

has no means therefore of discerning the globe from

the cube.

The most he could do, would be to reason in the following manner, after carefully examining the two bodies on all sides: "On whatever side I feel the globe," he would say, "I find it absolutely uniform; all its faces in regard to my touch are the same; one of these bodies, on whatever side I examine it, presents the same figure, and the same face; conse quently it must be the globe." But is not this reasoning, which supposes a sort of analogy between the sense of touching and that of seeing, rather too learned for a man born blind? It could only be expected from a Saunderson. But it would be improper here to enter into farther details respecting this question, which has been discussed by Molyneux, Locke, and the greater part of the modern metaphysicians.

What was observed in regard to the blind man, restored to sight by the celebrated Cheselden, has since confirmed the justness of the solution given by Locke and Molyneux.

When this man, who had been born blind, recovered his sight, the impressions he experienced, immediately after the operation, were carefully observed; and the following is a short account of them.

He

When he began to see, he at first imagined that all objects touched his eyes, as those with which he was acquainted by feeling touched his skin. knew no figure, and was incapable of distinguishing one body from another. He had an idea that soft and polished bodies, which affected his sense of touching in an agreeable manner, ought to affect his eyes in the same way; and he was much surprised to find that these two things had no sort of connection. In a word, some months