incidence. If it falls on a transparent medium, it is refracted or bent towards the perpendicular if the medium be denser than that it passed through before-from the perpendicular, if it be rarer. If the ray falls at right angles, no refraction occurs. The degree of refraction varies with the density of the medium, and also with obliquity with which the ray falls; if, therefore, a pencil of ray falls on a convex lens, the central ray will pass through unaffected, and those farthest from the centre will be most refracted, as they fall with greatest obliquity upon the lens. When the rays emerge from a biconvex lens into a rarer medium they will converge towards the central ray; and at this point, termed the focus, an image of the object from which the rays were originally given off will be formed. Six kinds of lenses are described, the first 3 of those below named being convergent or collective-the last 3 being divergent or dispersive. They are as follows: bi-convex, plano-convex, meniscus; bi-concave, plano-concave, concavo-conThe central line passing through them is termed the axis. The application of these laws of light to the eye is clearly stated by Todd and Bowman:

vex.

"If a luminous object, as the flame of a candle, be placed eight or ten inches in front of the organ, some rays fall on the sclerotic, and are reflected; the more central ones fall on the cornea; some are reflected, and others pass through it, are slightly converged by it, and enter the aqueous humour, which being probably of the same refracting power, does not alter their course. Passing onwards, some meet the iris and are absorbed or reflected by it, whilst others advance through the pupil. Thus, rays falling on a large extent of the cornea, are converged so as to fall on the lens. By the convexity of the surface of the lens, as well as by the greater density of that body towards its centre, this convergence is much increased. Lastly, by their passage

into the rarer medium of the vitreous humour, the rays

are further converged by the refraction of each ray from the perpendicular to the point of incidence, and the several pencils which they form are brought to as many foci in the retina. And still further, the rays from the opposite points of the luminous object, by reason of the change

The Course of the Rays through the Eye.

The rays from the object AB are refracted by the cornea CC, the aqueous humour, and still more by the lens EE and the vitreous humour behind it. They are thus collected to points at a and b, and the retina F being here situated, perfect pictures of these points, though reversed, are seen. If the retina be, however, in front (H), or behind (G) these points of convergence, luminous circles instead of points will be seen at c and f, or e and o, and no defined image will be produced.

of direction which they undergo through these successive refractions, cross one another (the angle of crossing being called the "visual angle"), and thus the image of the flame on the retina appears inverted." A careful study of the foregoing figure from Müller will make these principles intelligible.

The Adaptation to vision at various distances is one of the most remarkable properties the eye possesses. It has been supposed to be due to the recti muscles compressing the eye-ball, and making the cornea more convex when looking at near objects, or to the lens being moved forwards by the contraction of the iris or ciliary processes. The adjustment, is, however, accomplished

by the ciliary muscle moving forwards the lens, which afterwards falls backwards by the elasticity of its " suspensory ligament," or the anterior wall of the canal of Petit. The eye is therefore in a state of repose when viewing distant objects, but a muscular effort is required to look at a near one. This power is impaired by the action of belladonna or disease affecting the oculo-motor nerve, and is lost when the lens is extracted-to which statement, however, I have known of an exception.

Myopia, or near-sightedness, depends on too great refracting power, so that the image is formed in front of the retina; it occurs mostly in youth, but does not disappear as age advances, as commonly supposed. Concave glasses cause the image to be thrown farther back, but they tend to increase the defect.

Presbyopia, or far-sightedness, the opposite defect, in which near objects are not perceived, depends on too great flatness of the cornea or lens, a result of old age. Convex glasses remedy it by increasing the convergence of the rays; but if not carefully adjusted they may produce the opposite defect, and as the two eyes often differ in their refracting power, it is often impossible to make a pair of spectacles suit exactly. The diagram on page 375 will enable the student to comprehend the physics of long and short-sightedness.

Spherical Aberration, which should result from the fact before stated, that rays falling on a convex lens must be unequally refracted, is remedied by the iris, which, like the diaphragm of a telescope, shuts out the circumferential rays, and the density and refracting power of the lens increasing towards the centre. The iris also is of use in shading the retina from too strong light, and it acts in reflex obedience to that sensitive surface. In amaurosis, when the retina is insensible, the pupil dilates.

Chromatic Aberration is a fault due to the partial decomposition of the light while passing through a con

vex lens, but it is obviated in the eye and optical instruments modelled after it, by the varying densities of the media through which a ray necessarily passes. Sir D. Brewster believes this colouring does occur in the eye, aud it certainly does when fatigue, belladonna, or pressure on the eye-ball deranges its exquisite mechanism. Some of the rays of light which impinge on the retina become reflected therefrom, as the lustre at the back of the eye in some people shows; but in many animals this occurs to a great extent, as there is the mirror-like tapetum lucidum outside the retina. The images formed in Purkinje's catoptric test for cataract are due to reflexion. That valuable method of ascertaining the presence of cataract or glaucoma is performed as follows: the pupil having been dilated by belladonna, the patient is placed in a dark room; a candle with a steady flame is held before the eye, and if it be healthy, 3 images of the candle are seen the first erect on the cornea, the second reversed on the front of the lens, and the third erect behind it. When the candle is moved the erect images move in the same direction, the inverted one in the opposite direction. Cataract removes the middle or inverted one, and renders dim the deep one; while glaucoma renders more evident the deepest image. If the sclerotic and choroid be removed from the back of the eye, an image of any object placed before it is thrown on the retina, as on the screen of a camera obscura; but this probably does not occur in the living eye, as then the retina is as clear as glass.

As regards the real percipient of the optical image, the opinion that Jacob's layer has this function is gradually gaining ground. It has been urged by Donders, who has convinced himself that the yellow spot is also perceptive. Prof. Draper thus expresses himself: "Considering, therefore, the retina as typically composed of three layers-one of tubules, one of vesicles, and one of granules, and these in health being perfectly transparent,

the luminous beams pass through them, just as they do through the atmosphere, without exerting the slightest effect; and as, when those rays strike the opaque surface of the earth, or are absorbed by the sea, heat is disengaged and effects ensue, so likewise when they have reached the black pigment, the changes I have been designating arise. The vesicular layer undergoes rapid metamorphosis, the effect of that change is transmitted by the tubular layer, and in the granular the germs are constantly arising from which the waste of the middle layer is repaired. So, therefore, the tubular layer is for conduction, the vesicular layer for waste, the granular layer for repair; and now appears the significance of the construction and proximity of the choroid coat, for the waste of the vesicular layer cannot occur save under the oxidizing influence of the arterial blood, nor can the nutrition of the granular layer be accomplished, except under the same condition. Moreover, the resulting products of waste require to be quickly removed, and it is not possible to conceive the construction of an arrangement better adapted for this triple object than that which the choroid presents." The retina of the cuttle-fish consists of two layers, with the pigment interposed.

That images endure for some time is shown by a lighted stick, if rapidly whisked round, appearing as a luminous circle, and by the closure of the lids, in the rapid movement of vision, not interfering with continuous vision. The image remains on the retina about of a second, more or less according to the time it has been previously applied and to its intensity. Ocular spectra, or after-sights, are due to the image still persisting after the object has been withdrawn from the field of vision-or may be due to centric causes, which, as is the case with other special senses, will give rise to subjective sensations. Spectra are usually of the complementary colours-thus, green, if the image was red; yellow, if violet; orange, if blue. The accompanying