There is a scale of degrees from the most perfect opacity, as in a piece of clay, to the most perfect transparency, as in air. According as bodies become transparent they cease to be visible.

The transparency of air is not absolutely perfect; that is to say, light in passing through the atmosphere is to a certain small extent arrested, and a portion reflected, so as to make the mass faintly visible to the eye. When we look up into the sky through a cloudless atmosphere, all the illumination received from the surface is light reflected by the atmosphere itself. Liquids are still less transparent; although they transmit light so as to show objects beyond them, they also reflect a sufficient portion to be themselves visible. Light falling upon the surface of water is dealt with in three different ways. One portion passes through, a second is reflected as from a mirror, a third very small portion is absorbed and radiated anew, so as to make the surface visible as a surface. The same threefold action obtains in transparent solids, as glass, crystal, &c. It is to be remarked of solid bodies that they are almost all transparent to a certain small depth, as shown by holding up their plates or lamina to the light. Gold leaf, for example, permits the passage of light; and any other metal, if similarly attenuated, would show the same effect. There is, however, in this case, an important difference to be noted, inasmuch as objects are not distinctly seen, although light is transmitted; hence the name 'translucent' is applied to the case to distinguish it from proper transparency. There may be something more than a difference of degree between the two actions.

Opaque bodies may diffuse much light or little some substances, such as chalk and sea foam, emit a large body of light; charcoal is remarkable for absorbing without re-emission the sun's rays. This is the ordinary, perhaps not the full, explanation of white and black, the one implying a surface which emits a large portion of the rays of visibility, the other few or none.

Besides that difference of action which makes white and black, and the intermediate shades of grey, there is a difference

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in the texture of surfaces, giving birth to what we recognise as colour. Upon what peculiarity of surface the difference between, for example, red and blue, depends, we cannot at present explain. But this fact of colour is one among the many distinctions presented by the various materials of the globe. Along with colour a substance may have more or less of the property that decides between white and black, namely, copiousness of radiation. This makes richness of colour, as in the difference between new and faded colours, between turkey red and dull brick clay of a similar hue.

Bodies that are translucent to a certain depth have from that circumstance a distinct appearance, named their lustre. The effect of this property on the sense I shall discuss when we come to the Sensations of Sight.

Mineral bodies present all varieties of light, colour, and lustre, but the prevailing tone of rocks and soils is some shade of grey. The reddish tint of clays and sandstones is chiefly due to the prevalence of iron. Vegetation yields the greenness of the leaf, and the variegated tints of the flower. Animal bodies present new and distinct varieties.

2. We come next to consider the organ of sight, the Eye. 'Besides the structures which compose the globe of the eye, and constitute it an optical instrument, there are certain external accessory parts, which protect that organ, and are intimately connected with the proper performance of its functions These are known as the 'appendages of the eye,' (they have been named likewise tutamina oculi); and they include the eyebrows, the eyelids, the organs for secreting the sebaceous (or oily) matter, and the tears, together with the canals by which the latter fluid is conveyed to the nose.'

'The eyebrows are arched ridges, surmounting on each side the upper border of the orbit, and forming a boundary between the forehead and the upper eyelid. They consist of thick integument, studded with stiff, obliquely set hairs, under which lies some fat, with part of the two muscles named respectively the orbicular muscle of the eyelids and the corrugator of the eyebrows.' By this last-named muscle the eyebrows are drawn together, and at the same time downwards, so as to give

the frowning appearance of the eye; the opposite action of lifting and separating the eyebrows is performed by a muscle lying beneath the skin of the head termed the occipito-frontalis. In regulating the admission of light to the eye, and in the expression of the passions, these two muscles are called into play; the one is stimulated in various forms of pain and displeasure, the other in an opposite class of feelings.

'The eyelids are two thin moveable folds placed in front of each eye, and calculated to conceal it, or leave it exposed, as occasion may require. The upper lid is larger and more moveable than the lower, and has a muscle (levator palpebræ superioris) exclusively intended for its elevation. Descending below the middle of the eye, the upper lid covers the transparent part of the organ; and the eye is opened, or rather the lids are separated, by the elevation of the upper one under the influence of the muscle referred to. The eyelids are joined at the outer and inner angles of the eye; the interval between the angles varies in length in different persons, and, according to its extent, (the size of the globe being nearly the same,) gives the appearance of a larger or a smaller eye. At the outer angle, which is more acute than the inner, the lids are in close contact with the eyeball; but at the inner angle, the caruncula lachrymalis (a small red conical body) intervenes. The free margins of the lids are straight, so that they leave between them, when approximated, merely a transverse chink. The greater part of the edge is flattened, but towards the inner angle it is rounded off for a short space; and where the two differently formed parts join, there exists on each lid a slight conical elevation, the apex of which is pierced by the aperture of the corresponding lachrymal duct.'-QUAIN, p. 903.

The lachrymal apparatus is constituted by the following assemblage of parts-viz., the gland by which the tears are secreted at the outer side of the orbit; the two canals into which the fluid is received near the inner angles; and the sac with the duct continued from it, through which the tears pass to the interior of the nose. The description of these parts need not be quoted in detail here. Suffice it to say that the

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tears are secreted by the lachrymal gland, and poured out from the eyelids upon the eyeball; the washings afterwards running into the lachrymal sac, and thence away by the nose.

The parts now dwelt upon are not so much concerned in vision, as in expression and other functions auxiliary to vision. Though not directly bearing on the object of the present section, they will be of importance when we come to consider the emotions and their outward display. From them we now turn to the ball or globe of the eye.

'The globe, or ball of the eye, is placed in the fore part of the orbital cavity, fixed principally by its connexion with the optic nerve behind, and the muscles with the eyelids in front, but capable of changing its position within certain limits. The recti and obliqui muscles closely surround the greater part of the eyeball; the lids, with the caruncle and its semilunar membrane, are in contact with it in front; and behind, it is supported by a quantity of loose fat. The form of the eyeball is irregularly spheroidal; and, when viewed in profile, is found to be composed of segments of two spheres, of which the anterior is the smaller and more prominent; hence the diameter taken from before backwards exceeds the transverse diameter by about a line. The segment of the larger sphere corresponds to the sclerotic coat, and the portion of the smaller sphere to the cornea.'

Except when certain muscles are in action, the axes of the eyes are nearly parallel; the optic nerves, on the contrary, diverge considerably from one another, and consequently each nerve enters the corresponding eye a little to the inner or nasal side of the axis of the globe.

'The eyeball is composed of several investing membranes, concentrically arranged, and of certain fluid and solid parts contained within them. The membranes, not one of which forms a complete coat to the eye, are the conjunctiva, sclerotica, cornea, choroid, iris, retina, membrane of the aqueous humour, capsule of the lens, and hyaloid membrane. The parts

enclosed are the aqueous and vitreous humours, and the crystalline lens.'

The conjunctiva is more an appendage of the eye than a

portion of the globe.

It is a thin, transparent membrane

covering only the front or visible portion of the ball, and

reflected on it from the interior of the eyelids, of which it is the lining mucous membrane. Over the clear and bulging portion of the eye it is perfectly transparent, and adheres closely to the surface; on the parts surrounding the clear

*Horizontal section of the right eye, with two of the muscles,-the external and internal recti,-and the optic nerve. a. Aqueous humour. b. Crystalline lens. c. Vitreous humour. 1. Conjunctiva. 2. Sclerotica. 3. Cornea. 4. Choroid. 5. Canal of Fontana. 6. Ciliary processes. 7. Iris. 8. Retina. 9. Hyaloid membrane. 10. Zone of Zinn, or ciliary processes of the hyaloid. II Membrane of aqueous humour.-(WHARTON JONES on the Eye.)