the mind therefore receives the same idea from the retina of both eyes, and conceives therefore of only one object.

But while the vision thus accords with fact, and is strictly accurate, there are other cases when it is not so. Thus, if two coloured wafers be placed on a sheet of white paper, at the distance of three inches, and a person look at the left hand wafer with the right eye, at the distance of about eleven or twelve inches, taking care to keep the eye straight above the wafers, and the line which joins the eye parallel to the line which joins the wafers, and if then the left eye be closed, the right hand wafer will no longer be visible. The same effect will follow if the right eye be closed, and the right hand wafer be looked at with the left eye. The image of the invisible wafer has in each case fallen where the optic nerve enters the eye and expands itself to form the retina.

But though a common light makes no impression on this part of the eye, a very strong one does, and even if candles be used, or highly luminous bodies instead of wafers, the body does not wholly disappear, but leaves behind a faint cloudy light, without, however, giving any thing like an image of the object from which the light proceeds.

If the objects are white wafers on a black ground, the white wafer absolutely disappears, and the space it

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covers seems completely black. But here the perfection of the work of God is apparent. Though the base of the optic nerve is insensible to light that falls directly on it, yet it is susceptible of luminous impressions from the surrounding parts, and the consequence is, that when the wafer disappears, the spot it occupied, instead of being black, has always the same colour as the ground on which the wafer is laid; it is white on a white ground, and red when it is placed on a red ground. Thus the insensible spot on the retina is stimulated by a borrowed light, and the apparent defect so completely removed, that its occurrence can only be determined by the experiment already described.

In addition to these facts, there is another truly astounding. It has been disclosed by modern optical discoveries, that every point of a medium through which a ray of light passes, is affected by successive periodical movements, recurring regularly at equal intervals, no less than five hundred millions of millions of times in a single second. It is by such movements, communicated to the nerves of our eyes, that, according to Sir John Herschel, we see.

We are also told by Dr. Young, that when we see the following colours, our eyes are affected in a second, or while we can pronounce one as follows:-In red,

482 millions of millions of times; in yellow, 542 millions of millions of times; and in violet, 707 millions of millions of times.

In closing these general remarks on the eye, an ancient fable may be related, which is not without a good moral.

Alexander the Great, it is said, traversing dreary deserts, came at last to a small rivulet whose waters glided peaceably along. Yet tranquil and delightful as was this spot, it had no charms for him whose breast was swayed by ambition, and who cared not what sacrifices were made to gratify the passions of his soul. At length, overcome by hunger and fatigue, he was compelled to stop; when attracted by a river on the banks of which he took his seat, he thought it must flow from some very rich and happy country, and determined to proceed thither.

Following the course of the river, he arrived, says the tale, at the gates of Paradise. Here he knocked, but in vain, for as he stated his character and rank, the answer was given, "None but the just can enter here." He now addressed an attendant spirit, saying, "Since you will not admit me, give me at least something, that I may show an astonished and admiring world that I have been where no mortal has ever been before me." " Here,

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madman," said the spirit, "here is something for thee. It may cure the maladies of thy distempered soul. One glance at it may teach thee more wisdom than thou hast hitherto derived from all thy former instructors. Now go thy ways."

Alexander received the gift with avidity, and repaired to his tent; but what was his surprise and confusion to find that it was only the fragment of a human skull! Enraged and disappointed, he threw it on the ground. “Great king,” said a learned man who happened to be present, "do not despise this gift. Despicable as it appears in thine eyes, it yet possesses some extraordinary qualities, of which thou mayest soon be convinced, if thou wilt order it to be weighed against gold or silver." Alexander directed it to be done. A pair of scales was brought. The skull was placed in one, and a quantity of gold in the other, when, to the astonishment of the beholders, the skull overbalanced the gold. More gold was added, still the skull was heavier. In short, the more gold there was put in the one scale, the lower sunk that which contained the skull.

"Strange!" exclaimed Alexander, "that so small a portion of matter should outweigh so large a mass of gold! Is there nothing that will counterpoise it?"

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'Yes," answered the philosophers, " a very little matter

will do it." They then took some earth, and covered the skull with it, when immediately the gold fell, and the opposite scale ascended. Much astonished at the result, Alexander inquired: "Can you explain this mystery?" "Great king," said the sages, "this fragment is the socket of a human eye, which, though small in compass, is yet unbounded in its desire. The more it has the more it craves. Neither gold nor silver, nor any other earthly possession, can ever satisfy it. But when it is once laid in the grave, and covered with a little earth, there is an end to its lust and ambition."