10. The interior of the eyeball is filled with three liquids called humours separated from each other by delicate membranes. These, shown in figure 2, are called from their nature the aqueous, the crystalline, and the vitreous humours. They serve as a compound optical glass, to collect into a small space the light coming from objects, and so form a minute picture, m m, of external scenery. The explanation of the mode in which this is done belongs to the subject of optics, but the

Fig. 2.

same effect, though produced in a different manner, may be seen in the common convex mirror used as an ornament in rooms, and the truth of it

may be proved experimentally. If the back part of the eye of an ox recently killed be carefully removed, a minute inverted image of any object to which the pupil is presented, will be seen at the back. The experiment here mentioned shows that up to this point the process of vision is wholly mechanical, and independent of life and volition. The crystalline humour (c, c') is the most powerful in its action on light. It is enclosed in a little sack, and is held suspended between the other two by a membrane, called the ciliary process (b, b'). Loss of vision, arising from the thickening and consequent opacity of this humour, is known as cataract, the only mode of curing which is by removing the defective crystalline lens, and supplying the consequent want of power in the eye by a stronglyrefracting glass lens in front.

11. The part of the inner surface upon which the little picture is formed is covered over with a delicate network, called from its nature the retina, and all the rest of the interior is blackened by a lining called the black pigment (pigmentum nigrum), intended to prevent any internal reflections of light. The retina is that which brings the mechanically-produced picture (m, m') of the object (l, l') into communication with the nervous

and mental acts of sensation; it is, in fact, a delicate ramification of a nerve called the optic nerve (n), which enters near the back of the eye. This nerve is not sensitive to light until it has branched out, so that the point at which it enters is called the dark point (punctum cæcum). This fact has been referred to, to explain a singular effect noticed by astronomers; if you look intently at a particular star, it sometimes becomes invisible; but by glancing the eye slightly aside, the light of the star falls upon a sensitive part of the retina and produces vision.

12. By the sense of sight we learn the colours and outlines of bodies, and all the modifications which light undergoes in transmission or reflection. It does not teach us the shape or size or distance of objects, though it greatly assists the mind in estimating all these. From the experience already recorded, of the boy with restored vision, as well as from our own daily observation, we must conclude that a solid body is not by the sight alone distinguishable from a flat surface coloured to represent it. The reason of which is that, in both, the eye perceives the same gradations of colour, and these alone it is capable of estimating. It is not, in fact, in any case the body seen which affects the organ, but the light reflected from it; so that if we can by colouring make a flat surface reflect light in the same manner as a solid, it must produce the same sensation.

13. As regards measurement of size and distance, the incapacity of the eye without training is very manifest. We find that an object a foot high placed near the eye has the same effect on the sentient nerve as a man standing at the distance of a mile. It is true we judge of distances and sizes now with tolerable accuracy, because the vision has been educated and is generally informed by the mind, either as to the natural size from which it judges the distance, or the probable distance from which it estimates the size. Gunners who have been well exercised in the use of their weapons can in this way measure the distance of an unapproachable object with considerable accuracy.

14. We must not consider the organ of vision defec

tive because the knowledge it gives is limited in kind. It is perfect in doing the work which nature has assigned to it, and the Almighty has given to us an intellect to guide its search for information, and other senses to supply different kinds of knowledge. Its tidings are in fact never untrue, but they are often partial, and therefore likely to mislead, unless we understand the exact amount of dependence which can be placed upon its indications. Indeed the limited power of our sense of sight, so far from being a defect, is the source of some of our most refined enjoyments, for to it we owe the glorious art of the painter. Did the eye give a real knowledge of size and distance, the pleasing delusions of painting would be impossible. Nobody could then for a moment imagine that a speck upon a canvas represented a man or a horse. The reality of appearance which produces the charm in viewing a picture could not exist unless shades of colour made the same impression on the organ as a solid form, and decrease of size in perspective gave the full effect of an increase of distance in the object.

15. Having thus shown what the eye can do of itself and what it cannot, let us for a moment consider what it can accomplish when trained by experience. It is the most powerful and glorious organ with which the Creator has endowed his creatures. It delights the mind with the order and beauty of the universe, dis-. closing to us by a single glance an infinity of objects with their innumerable hues and variety of groupings. It penetrates space which we cannot traverse, bringing before us in one view the near and the remote, the large masses and the minute workmanship of nature. It guides our steps in safety, and warns us of danger before its approach. Above all, it opens to the cultivated mind a vast mine of wealth, and source of neverfailing delight, in books which contain the learning and wisdom of ages.

16. How pitiable is the condition of a human being who has never enjoyed this wonderful gift of sight, or has been by accident or disease deprived of it! Groping under the noonday sun as at midnight; his world limited

to the grasp of his arms; never beholding the beautiful panorama of nature, or, drawing wisdom from the printed page, and exposed to numberless dangers without a friendly monitor to warn him of their approach. Well may our great poet bewail his loss:

As the wakeful bird

Sings darkling and in shadiest covert hid

Tunes her nocturnal note. Thus with the year
Seasons return, but not to me returns

Day, or the sweet approach of even or morn,
Or sight of vernal bloom, or summer's rose,
Or flocks or herds or human face divine;
But clouds instead, and ever-during dark
Surrounds me, from the cheerful ways of men
Cut off; and for the book of knowledge fair
Presented with a universal blank

Of nature's works, to me expunged and rased,
And wisdom at one entrance quite shut out."

17. It has been computed that there are in this country 25,000 unfortunate fellow-creatures deprived of the blessing of sight, with "wisdom at one entrance quite shut out." How considerate ought we to be for their unhappy condition! How anxious to open through the other senses an access to their minds! How grateful should we feel to the Father of Good, who has both clothed the earth in beauty and enabled us with unimpaired senses to behold his glorious works!

LESSON II.

THE USE OF THE SENSES-continued.

"And the Lord said unto him, Who hath made man's mouth? or who maketh the dumb, or deaf, or the seeing, or the blind? have not I the Lord ?"

1. TOUCH.-This sense is not like the others confined to any particular organ, but is diffused over the whole surface of our bodies. It is, however, much more acute in some parts of the frame than in others, and particularly so at the ends of the fingers, which, by their flexibility, are so well adapted for applying it to the various objects requiring examination. It generally, in conveying

information, acts in conjunction with the muscular sense, from which we obtain the notion of weight, solidity, hardness, and softness, and probably of size. We can scarcely feel an object without moving the hand or fingers, so that it is not easy to distinguish between what we owe to each sense. Touch, however, gives us no farther information than that of smoothness, or inequality of surface, and heat and cold. All other sensations commonly ascribed to touch are the impressions upon the muscular sense. Weight is that which requires exertion to move, force that which overcomes our muscular resistance or calls for its exercise; hardness and softness express the degrees of resistance to muscular pressure; and the notion of size may result from the different amounts of muscular movement required to pass over unequal spaces.

2. The action of this sense is slow in comparison with that of others, since every part of a surface requires to be felt in succession before we can receive the same information which sight gives instantaneously, from its including the whole in one view. If the objects to be studied be large or numerous, this examination in succession instead of at once, as in the case of vision, must render it difficult to get a notion of general arrangement and order; it must also render comparison more difficult, as two objects can rarely be felt at the same time. There are, however, many instances on record of the intellect having accomplished works of great difficulty aided by this sense alone. The blind Saunderson, the friend of Newton, became a professor of mathematics in the University of Cambridge, and lectured upon the motions and order of those millions of the sphere which he could not behold, and upon those laws of light which never, alas! could affect his mind. The roads through Derbyshire were constructed by Metcalf, a blind engineer; the blind traveller Lieut. Holman has visited and described half the countries of the world; and many other instances might be mentioned in which the mind has shown itself superior to bodily infirmities. 3. The delicacy and precision of touch acquired by some workmen, engravers for instance, by constant