rays proceeding from each luminous point of the object independently.

Permeability of Bodies to Light.-All substances are probably pervious to light in different degrees and to different depths. If the substance be one which allows no rays of light to pass through it, it is said to be opaque; if it be one which admits of light passing through its substance, or transmits rays sufficient to allow objects to be seen through it, it is termed transparent; if the substance be one like thin porcelain, which permits the passage of light through it, but does not allow objects to be seen through it, it is described as being translucent. Shadows result from opaque bodies intercepting the passage of light. All bodies, whether opaque or transparent, vary in aspect and colour according as they vary in reflecting, absorbing, or decomposing the light which falls on them.

Reflection of Light.-All bodies, whether opaque or transparent, with smoothly-polished surfaces, under certain conditions turn. away the rays of light which fall on them from their original direction. This is termed reflection of light. There are two general laws of reflection of light. The first is that whatever may be the angle which a ray impinging on a polished surface forms with the normal, or perpendicular, to that surface, the angle at which it is turned away from that normal will be the same; in other words the angle of reflection is equal to the angle of incidence, and on the opposite side of the normal (see fig. 10). The second law is that the plane in which the incident ray is found will be the same as the plane in which the reflected ray is found, or in other words, the plane of incidence coincides with the plane of reflection. All the phenomena of reflection of rays of light from polished sur

faces, whether plane or curved, take place in accordance with these laws. If the polished surface be either level or have a regular curvature, the reflected rays of light produce images of the objects from which the rays have proceeded. If the surface be roughened,

FP', perpendicular, or normal, to ns, the reflecting surface; IR, incident ray; RR. reflected ray; PP'R, the angle of reflection, is equal to PPI, the ang.e of incidence, and is on the opposite side of the normal, Pr'.

there will still be reflection of light, but the reflected rays are irregularly dispersed, and no images are produced.

Refraction of Light.-Rays of light proceed in straight lines so long as the medium through which they are travelling is of uniform density. When a ray passes obliquely from a rarer into a

R

R. M.

P

FIG. 11.-REFRACTION OF LIGHT.

PP'P, perpendicu'ar, or normal, to RM, one surface of a refracting medium, such as a piece of glass with opposite parallel surfaces; IR, incident ray in air; KR, incident ray refracted towards the perpendicular in passing through RM; ER, emergent ray refracted from the perpendicular in passing out of RM into air.

denser medium, it is bent or refracted towards a line drawn perpendicularly to the surface of this medium at the point of incidence; conversely, on passing obliquely from a denser into a rarer medium, it is refracted from a line drawn perpendicularly to its surface (see

fig. 11). This change of direction commences at the surface of separation of the two media. When a ray of light passes through media of different densities perpendicularly to the surfaces where these media are in contact with one another, the ray travels onwards in one and the same straight line. In accordance with this rule, a ray of light passing through air and impinging on the surface of a piece of glass perpendicularly to the point of incidence, passes on uncharged in direction; but if it fall on the surface with a slanting direction, it is refracted in passing through the glass towards the perpendicular to the surface at its point of incidence. Any substance, whether liquid or solid, through which light can pass, will produce a similar effect to that produced by the glass, but the refraction will vary in degree in substances of different kinds. The deviation of rays of light from their original direction on passing obliquely from one into another medium of different density takes place according to fixed laws, and the investigation of these laws, and of the phenomena which result from them, constitute the

OA

FIG. 12. INVERSION OF RETINAL IMAGES.

Q, object; a, b, and c, upper, middle, and lower diverging pencils of rays emanating from Q; OA, optic axis; D, centre of pupil, or decussating point of the central axes of a'l the cones, from whence they continue to diverge, while the marginal rays converge towards them (as seen at a', b', and c′), and finally depict the inverted image (1) on the retina.

branch of optics generally termed dioptrics. The three following laws are constant in all cases of refraction:-(1.) The angle formed by an incident ray of light with the perpendicular to the surface, or the angle of incidence, and the angle formed by the refracted ray with the perpendicular, or the angle of refraction, are in the same plane. (2.) The incident ray and the refracted ray are always on opposite sides of the perpendicular. (3.) Whatever the inclination of the incident ray to the surface, the sine of the angle of incidence has a constant ratio to the sine of the angle of refraction. These laws apply to curved surfaces equally with plane surfaces, and hence, when the form of surface and nature of a refracting medium are known, the path of any refracted ray can always be determined.

Law of Visible Direction. Each point of an object is seen in a line perpendicular, or nearly so, to the point of the retina which its image impinges.

Inversion of Images on the Retina. The pictures formed on the retina of external objects are inverted and curved, owing to the action of the optical apparatus of the eye, together with the concave

B

form of the retinal receiving surface (see fig. 12). The mind, however, does not judge of the positions of objects, whether primary or reflected, according to the part of the retina on which their images happen to fall; if it did, the positions of the things would appear to change with changes in the position of the eyes looking at them. But the mind judges of the positions of objects by following, as it were, the directions of the axial rays proceeding to all the points of these objects from the parts of the retina on which the corresponding images of such points are pictured. Hence, though the images of objects looked at directly are inverted on the retina by the action of the refracting media of the eye, the mind, following the lines of light to their sources in accordance with the law of visible direction, sees them in their true positions. The images of objects below the level of the visual diameter are pictured in the upper retinal hemisphere; the images of objects above this

A'

A"

B"

B'

D

FIG. 13.-EXPLANATION OF VISUAL ANGLE.

Visual angles ABE, CDE, &c. CD, nearer to the eye than AB, though alike in size, has a larger visual ang'e and forms a larger retinal image. A'B", A'B', AB, though smaller than each other, have the same sized visual angle, and form images of the same size upen the retina. Distance and size together determine the magnitude of the visual angle, the size of the retinal image, and the apparent sizes of objects as seen by the observer.

line in the lower retinal hemisphere; they are equally reversed in the lateral portions of the retinal picture; but, nevertheless, all the objects are seen in their real positions and relations to each other. This equally applies to the reflected images of real objects: the reflected images are inverted on the retina, but they are seen truly in the forms in which they proceed from the reflecting surface.

Visual Angle. The visual angle is the angle included between two rays proceeding from the opposite extreme limits of an object looked at by the eye and meeting at a point within the eye. These rays, having met, cross each other and pass onward to assist in forming the image on the retina. The point at which they meet is known as the point of intersection,' or 'nodal point,' of the eye. The size of the visual angle depends on the linear dimensions of an object, and on the distance of the object from the eye. If the object be of a fixed size, the size of the angle under which it is

seen will vary inversely as its distance from the eye; if the distance be fixed, the size of the visual angle will vary in a direct ratio with the size of the object. The angle is similar on each side of the point of intersection- towards the object, the visual angle,' and towards the image of it on the retina, the retinal angle.' The expression that an object occupies so many degrees in the circumference of a circle of which the eye is the centre, or that it subtends an angle of so many degrees, has the same significance as the size of the visual angle of the object.'

The size of the image of an object formed on the retina varies as the retinal angle, and therefore as the visual angle varies under which the object is seen. The larger the visual angle, the larger the retinal image. If the size of an object remain the same, the visual angle it subtends is increased in proportion as it is brought nearer to the eye; and hence, the frequently observed approximation of printed letters by Hc. patients to their eyes, although the diffusion of rays about the retinal images and the strain on the accommodation are increased by the proceeding. A similar approximation of print generally takes place among amblyopic subjects. In both instances the retinal images are increased in size, and the area of sentient visual impression proportionally enlarged; and the advantages attending these results preponderate over the disadvantage of the loss of distinctness of outline, due to the diffusion of the marginal rays.

The retinal image will also vary in size as the distance from the point of intersection of the rays forming it to the plane of the retina varies. In a short, or hypermetropic eye, the distance from the nodal point to the retina will be less than it is in an elongated or myopic eye, and the rays proceeding to form the retinal image of an object, being sectionally interrupted in their course earlier in the former than in the latter case, the areal size of the image will be necessarily less in the former than it is in the latter instance.

If the position of the point of intersection in an eye is made to alter, the size of the retinal image will be altered also, but this can hardly happen except artificially by placing a convex or concave lens before the eye. If a convex lens be placed before an eye, the point of intersection will be caused to advance, and the retinal image will become enlarged: if a concave lens be similarly placed, the point of intersection will be caused to recede, and the retinal image will be lessened in size.

Field of Vision.-The term 'field of vision' signifies the whole of the space, including the objects comprised in it, which is perceptible to sight in one fixed position of the eye, or, in binocular vision, of the two eyes.

Monocular Field of Vision.-When a single eye is directly fixed in a given direction its horizontal limits of visual perception are comprised within an angle of about 123°.