accommodation, and in addition to this the formation of double images in a single eye, and the relation of retinal impressions to cerebral

FIG. 225.-PERIMETRIC CHART OF THE RIGHT EYE.

The thick line surrounds the area within which white is visible, the point C being fixed by the eye (right); the fine dotted line surrounds the area within which green is visible. The area of red would be somewhat larger, that of blue larger still, though not so large as that of white. B = the situation of the blind spot.

Two pinholes are pricked in a card at about one millimeter from each other; and the card being held in front of one eye, with the two pinholes side by side in front of the pupil, a needle, held vertically, is looked at through them.

(a) With the eye accommodated for the needle, the latter is seen single, and if it be brought gradually nearer to the eye, a point is

N

10

FIG. 226.-TO ILLUSTRATE SCHEINER'S EXPERIMENT. See text (a).

i

Vertical section of eyeball. N, Section of needle held horizontally. CC, Card pierced by two pinholes, one above the other, held vertically;

Or, horizontal section of eyeball. N, section of needle held vertically; CC, card held vertically, with pinholes right and left.

reached at which it can no longer be seen single by any effort of accommodation; the distance of the needle in front of the eye is now that of the near point of distinct vision. With the eye accommodated to a point either nearer or farther than the needle, the latter is seen double.

(b) If the eye is accommodated to a point beyond the needle, or, better still, if it is non-accommodated, two images will appear, the eye

being under-accommodated as regards the needle; these images will move towards each other as the needle is moved from the eye, will

FIG. 227.-TO ILLUSTRATE SCHEINER'S EXPERIMENT. See text (b).

Vertical section of eyeball and horizontal needle, or horizontal section of eyeball and vertical needle.

Blocking the upper hole in the card abolishes the retinal image i,, i.e. the apparent inferior needle in the field of vision; blocking the lower hole abolishes 2, i.e. the apparent superior needle.

enlarge and move away from each other as the needle is moved towards the eye. If the right-hand hole is covered, the apparent left-hand needle will disappear; and, vice versa, if the left-hand hole is blocked the apparent right-hand needle will be lost.

(c) If the eye is accommodated to a point nearer than the needle, it is over-accommodated as regards the needle; two images will be apparent, and if the needle be moved to and from the eye, they will move to and from each other. Blocking the right-hand hole effaces the apparent right-hand needle, blocking the left-hand hole effaces the apparent left-hand needle.

FIG. 228.-TO ILLUSTRATE SCHEINER'S EXPERIMENT. See text (c).

Vertical section of eyeball and horizontal needle, or horizontal section of eyeball and vertical needle.

Blocking the upper hole abolishes the retinal image i, i.e. the apparent superior needle; blocking the lower hole abolishes i,, i.e. the apparent inferior needle.

The whole series of observations may be repeated with the needle held horizontally, the card being held so that the holes are one above the other in front of the pupil.

Chromatic dispersion. The lens of the eye, like other lenses, refracts blue rays more strongly than red rays. Consequently the focus of blue is nearer to the lens than the focus of red. A spot of white light is surrounded by a blue halo if the eye is focussed to a point beyond the spot, by a red halo if it is focussed to a point nearer than the spot; to see these halos distinctly the light should be viewed through cobalt glass, so as to cut off the yellow and green, allowing only red and blue to pass.

A blue light and a red light at the same distance from the eye appear to be unequally distant; the red light requiring greater accommodation than the blue, appears to be the nearer of the two, nor can both lights be exactly focussed together upon the retina. Of a red and a blue surface side by side, the red appears to be the nearer.1 The difference is such that a normal eye focussed for parallel red rays is at the same time focussed for blue rays divergent from the points of a surface about 3 feet distant.

Astigmatism. The surface of the cornea is not perfectly spherical; the curvature of its vertical meridian is usually greater than that of its horizontal meridian, and the difference may be so pronounced as to interfere with the distinct focussing of objects. With a cornea of such curvatures (spoon-shaped'), a point of light cannot form a focal point upon the retina (hence the name astigmatism), but forms a linear focus; if the eye is adjusted so that the rays diverging from the point in the vertical plane (meridian of greater curvature) come to a

The curvature of the cornea is greater in the vertical meridian, v v v, than in the horizontal meridian, h h h. The point of light, P, consequently has a first linear focus at f, which is horizontal, and a second linear focus at f2, which is vertical. If, instead of P, the object were a vertical and a horizontal line crossed, the vertical line would be in focus at f, the horizontal line would be in focus at f. To bring the two lines into focus at the same time on the same plane it would be necessary to use a convex cylindrical glass to add to the horizontal convexity hhh, or a concave cylindrical glass to subtract from the vertical convexity v vv.

focus on the retina, then the rays diverging from the point in the horizontal plane (meridian of lesser curvature) will reach the retina before they have come to a focus (i.e. be under-focussed), and a horizontal linear focus will be formed; vice versa, if the eye is adjusted so that rays in the horizontal plane (meridian of lesser curvature) are focussed upon the retina, then the rays in the vertical plane will reach the retina after coming to a focus (i.e. be over-focussed), and a vertical linear focus will be formed. As shown in the diagram the horizontal linear focus, fi, is nearer than the vertical linear focus, f2.

Consider next the case of a series of points, viz. a line, vertical or horizontal. A person whose cornea is regularly astigmatic, as supposed above, can see distinctly either a horizontal or a vertical line, but not

This is not a complete explanation. Einthoven has shown that the effect is mainly dependent upon excentricity of the pupil. Most persons see red in front of blue, but some persons see blue in front of red.

'. p. 687.

both together. If the eye is focussed for a horizontal line it is underfocussed for a vertical line, if it is focussed for a vertical line it is overfocussed for a horizontal line; thus the astigmatic eye cannot be accurately focussed at one and the same time for a vertical and a horizontal line that cross each other; one or other must be indistinct (over- or under-focussed). A greater effort of accommodation being required to focus a vertical than a horizontal line, the former appears to be nearer to the observer than the latter-i.e. a vertical line appears to be in front of a horizontal line which it crosses in the same plane; further, as the eye has greater focussing power upon a horizontal than upon a vertical line, the former remains distinctly visible at a shorter distance from the eye than the latter.

Irradiation. The media of the eye are not perfectly transparent; the lens is not perfectly homogeneous. These causes contribute to the slight defect of focussing present even in the most normal eye, and give rise to a certain amount of dispersion or 'irradiation.'

The ophthalmoscope. The ophthalmoscope is used to examine the fundus of the eye, more particularly the optic disc and the retinal vessels, and to detect and estimate errors of refraction. For the examination of the retina the 'indirect' or the 'direct' method may be adopted the former is the more generally serviceable, and yields a reversed image of a considerable area of the retina magnified about five times; the latter is useful for more minute examination, and yields an erect image of a small area of the retina magnified about twenty times. To estimate refraction the direct method must be adopted.

An eye accommodated for a given point f' is equivalent to a lens with conjugate foci at the point f' and at the fovea f. An object in the plane f' has its real reversed image focussed small in the plane ƒ; conversely, an object in the plane ƒ (e.g. a retinal vessel) has its real reversed image focussed large in the plane f'. The conjugate focus f' can, by strong accommodation of the observed eye, be made to coincide with the near' point of that eye, and may be seen by an observing

FIG. 230.

eye at o.
The smallest distance at which an image in the plane ƒ'
can be seen clearly by an observer, is the near point of the observer's
Thus the minimum distance at which focussing is possible with
accommodation of the observed and observing eyes is about 10 inches.

If the converging power of the observed eye be increased by a convex lens (15 to 20 D) placed in front of it (accommodation being now unnecessary), the conjugate foci of the system will be as under, j' being

brought close to the eye, and a retinal vessel in the plane ƒ will have its real reversed image nearer to the eye in the plane f'. An observing eye placed at o (so that the distance of' is greater than that of the observer's near point) could see this image of the retinal vessels if the

FIG. 231.

retina were a source of light or lighted up so as to reflect sufficient light. This can be done. The fundus of the eye is made visible by rays reflected from a mirror, which light up its vessels, &c.; rays reflected from parts so illuminated emerge from the observed eye and form the image f', which is seen by the observing eye.

The main principle upon which the ophthalmoscope depends is thus that the line of vision of the observing eye shall be in the line of illumination. This is effected by means of the perforated mirror through which the observer looks while he directs light along his line of vision. This method of examination is known as the indirect method. The retinal object, a b, is illuminated by light reflected from a concave perforated mirror; rays reflected from the bright object, a b, are refracted

FIG. 232.-DIAGRAM TO SHOW HOW OBJECTS ON THE FUNDUS OCULI ARE ILLUMINATED

AND SEEN BY THE INDIRECT METHOD.

The lines of vision and of illumination are made to coincide by the perforated mirror, which is concave in order to concentrate the light.

by the observed eye and auxiliary lens to form a real reversed image, ba', which is looked at by the observing eye through a hole in the mirror (fig. 232).

The second method of examination is known as the direct method; the image thus obtained is a virtual erect image formed by rays as they emerge from the observed eye. In this case, to obtain any useful view, the observing and observed eyes must be as close as possible. As will be understood from the description of the three conditions of refraction that we may have to deal with, the direct method' is one of the means of detecting and estimating errors of refraction.

Rays emerging from any given point at the fundus of a normal eye