Binocular Field of Vision.-When both eyes are fixed on an object situated at such a distance in front of the observer that the visual axes of the two eyes are practically parallel with one another, the limiting lines of visual perception or luminous impression on each side of the face form an angle of about 90°, so that the general field of vision of the two eyes under these conditions has a horizontal limit of about 180°. The precise limits of the field of vision will vary with the amount of projection, and other peculiarities in form of the facial features. A certain central portion of the visual field is common to both eyes; the right and left temporal portions of the field are only proper to the right and left eyes respectively. The absence of this monocular part of the binocular field of vision is a serious drawback from the usefulness of a soldier in military service.

FIG. 14.-GENERAL, COMMON, AND PROPER FIELDS OF VISION.

A. Sketch showing the visual limitations, in a fixed position of the face, due to the projection of the principal features.

B. Diagram showing the outlines of the common, proper, and general fields of vision, with measurements. a, b,f, b, outline of general fie'd of vision; 1 and 2, right and left proper fields of vision; 3, common field of vision; aa, upper limit due to frontal projection; cc, limits due to ma'ar projections; ee, limit produced by labial projection; dd, by nasal projection; bbf, limits due to projections of cheeks.

Measurement of the Monocular Field of Vision.-Instruments specially constructed for examining and providing the data for mapping out a field of vision are called perimeters. In the absence of these special instruments, the extent and shape of the field of vision of an eye may be obtained by causing the patient to look at a given point, and, while the eye is fixed upon it, drawing an outline of the boundary of distinct vision in all directions around it. The most convenient plan is to place the patient with one eye covered, and the other free, at a distance of about two feet in front

of a black board placed perpendicularly to the line of sight. On this board, at a level with the eye under examination, a small cross in white chalk is drawn. The patient is desired to fix his eye on this mark. At the same time the surgeon, who must watch that the patient does not look away from the cross, holds a piece of chalk between his fingers of his right hand, and carries it from point to point over the board by slight quick movements of the hand, jotting down the points in various directions where it ceases to be seen. These points are now joined together by lines, and thus an outline of the shape of the field of vision is formed. If the map thus made be copied on paper, it can be retained for comparison with other diagrams to be made in a similar way on future occasions.

If it be important to examine the field of vision with greater precision, separate outlines can be obtained by a similar plan showing where vision sufficiently distinct to count fingers ceases, and more externally where imperfect vision or luminous impression ceases and complete absence of sight begins. Any loss of visual function in particular spots of the retina may also be traced and noted during the examination by moving the chalk slowly and carefully over the field, and directing the patient to mention whenever it disappears from view altogether or is only seen. obscurely. Particular irregularities of form, and limitations in extent, of the field of vision will often be rendered manifest by this mode of examination in cases of weak and defective vision depending on disorders of the optic nerve and retina.

When it is not required to make a picture of the field of vision but only to ascertain quickly the extent of the field, or whether there is any break in it in any given direction, the following method by the finger will answer the purpose readily and quickly. The surgeon, standing about two feet off, and face to face with the patient, desires him to close one eye, the right, for example, and at the same time closes his own left eye opposite to it. He now desires the patient to look steadily into his right eye, and while the two eyes, the one of the surgeon and the other of the patient, are thus directed to each other, the surgeon moves his forefinger from a central point midway between his own eye and that of the eye under observation, in all directions towards the limits of the field of vision. He is thus able to note the extent of the patient's range of vision, by comparing it with his own range. This ready method has the advantage of being capable of being put into execution anywhere without need of any appliances or previous preparation.

Movements of the Eye.-The external muscles connected with the globe of the eye cause it to revolve round an ideal centre—its centre of rotation. Consequently if the anterior aspect of the eye is caused to move in one direction, the posterior aspect of the eye will be caused to move in the opposite direction. The movements

of the eye are commonly described with reference to the movements of the anterior pole of the optic axis or central point of the cornea. When the central point of the cornea accords with the point of intersection of one line drawn transversely between the apices of the two angles of the palpebral aperture, and another line drawn vertically midway between its two extremities, or does so very closely, allowance being made for the slight angle formed by the intersection of the optic axis and visual axis, the eye is said to be directed straight forwards; when the centre of the cornea is above or below the horizontal line just named, the eye is said to be directed upwards or downwards respectively; and if to the inner or outer side of the vertical line just named, it is said to be turned inwards or outwards respectively. It is in accordance with these distinctions of position that when strabismus exists the dis

The arrows show the direction of the movements. The four recti muscles singly effect purely vertical and transverse movements (plain arrows), whi'e those in either diagonal line are produced by the joint action of two recti muscles (dotted arrows). Rotation to the right or left is effected by the oblique muscles (curved arrows).

placement of the deviated eye is said to be internal or convergent, external or divergent, and upwards or downwards. In normal binocular vision, when the two eyes are looking directly forwards at a distant object, the visual lines are practically parallel with one another, and perpendicular to an imaginary line joining the centres of rotation of the two eyes.

Field of View. The term 'field of view' signifies the space over which objects can be seen clearly by an eye when it moves round its centre of rotation to the extreme limits of which it is capable, the head of the observer at the same time remaining stationary. In binocular vision, the field' of view signifies the visual range of the two eyes under like conditions.

Ocular Movements affecting the Field of View.-Fig. 15 shows the manner in which the visual range is obtained, and indicates

the directions in which the movements affecting the field of view

occur.

Normal Range of Motion of the Eye consistent with Vision. I find that my own eye can turn through an angle of about 140° horizontally, and of 138° vertically, and perception of objects be retained. On my eye being directed to a point straight before it, it can turn from it 50° towards the nose and nearly 90 outwards in the horizontal plane, or through an angle of 48° from it upwards and 90° downwards. These measurements are not, however, universal; they are subject to variations according to individual circumstances, viz. to personal peculiarities in the shape and amount of projection of the parts near to the eye, as the nose, the margins of the orbit, the eyebrows, and other structures.

Objects on the Visual Field. The place and space occupied by objects in the field of vision are measured by the visual angles under which they are seen. The apparent size, or lineal measure, of an object is estimated by the size of the visual angle alone. To estimate the true size, the distance of the object, as well as the size of visual angle, must be known. Conversely, the true size of an object being known, the visual angle enables us to form a judgment of the distance at which it is placed from us. The apparent size, or lineal measure, is to be distinguished from the apparent superficial size, or measure of surface of an object. The lineal measure, as before mentioned, varies inversely as the distance of the object; the measure of surface varies in proportion to the squares of the lineal measure at different distances.

Illumination of Objects. The quantity of light received on a given object, or unit of surface, is reduced inversely as the square of the distance from the source of light to which the object is removed. Let a printed paragraph be placed at a certain distance from a lighted candle, and the paragraph be then removed to double the same distance from the light, the intensity of illumination of the paragraph in the more distant of the two positions will be only one-fourth of what it was in the nearer one; and the rule that applies to the whole paragraph equally applies to each letter of the print composing it.

Apparent Brightness of Objects at Different Distances in open Daylight. The vividness of the light under which objects appear in open daylight at different distances does not, however, vary with the distances at which they are respectively placed, excepting so far as those which are more remote may be affected by the thicker stratum of air through which they are seen, provided that all the objects. face in the same direction. Supposing the atmosphere to be perfectly clear and transparent, the apparent illumination of a series of targets, one more remote than the other but facing in the same direction, or of a line of men one in front of the other, will be exactly similar. A transverse section of the cone of rays entering

the pupil from each illumined point of the farther object, and therefore the total quantity of light emitted by the whole object, will vary inversely as the square of the distance to which the object is removed, but so also will the apparent area of the object. If one object be placed at double the distance of the other, the area of its image on the retina will be reduced to one-fourth of that of the nearer one, and so also will be the amount of light entering the eye from the more remote object, so that the effect as regards apparent brightness will necessarily remain the same. The area of the image on the retina is reduced in the same proportion as the quantity of light which forms it.

In fig. 16 the targets are represented as being at equal distances apart, and, as may be seen by the nearer targets which admit of observation in the drawing, each is one-fourth smaller than the target in front of it, and one-fourth larger than the next target beyond. As the amount of light received by the eye from each target varies in the same proportion as the visual angle under

FIG. 16.-APPEARANCES OF OBJECTS PLACED AT EQUAL INTERVALS APART. GL ground-line, on which the observer is supposed to stand; SP, standing-point; HL horizontal line on a level with the observer's eye, all round: Ps, point of sight or position of observer's ere on the HL; VI. vanishing lines drawn from the sa ient or special points of all objects into the point of sight, determining their diminution in size as the distance increases. The observer is represented by the perpendicular.

which it is seen the apparent brightness of the object is unchanged. The same observation applies to the human figure under like circumstances.

Motion of Objects in the Field of Vision. The movement of any visible object across the visual field, or the change of its position relatively to the positions of other objects, is, of course, accompanied by a similar movement of its image across the retinal picture. The movement which is thus rendered visible is called its apparent motion. If the object move in a direct line towards the centre of the eye, no change occurs in the position of the image of the object, and its movement is not apparent. The extent of apparent motion of an object is measured by its angular motion; that is, by the angle formed by two lines drawn from the point of visible departure, and point of visible arrest of motion, of the object, to the point of intersection within the eye. The real movement of an object, in respect to direction and extent, is estimated by other means-by its change of apparent size as its