130°, when it has attained its minimum of distinctness, and beyond that point it ceases to be visible.

In the last-mentioned portion of the experiment the image of the object moves through the retinal angle pk n or o km, and therefore through a space corresponding to the radius of the circle of "visual duality" minus that of the circle of visual unity. If now, instead of a single eye, both be used in this experiment, and the produced optic axes convergent upon an object situated in the common axis be moved 3° towards the visual base," the image of the object will move in both eyes through the segment of a circle lo, and produce but a single perception in the sensorium. If, while the position of the object remains unaltered, the axes still continue to move in the same direction, till they intersect in a point in the common axis 37° nearer the visual base, the images of the object will travel through segments o m, and produce a double sensorial impression.

Second experiment.—If three small bodies be placed at a, b, and c (figure 5)

6 The terms "visual base" and "common axis" were first used by Dr. Wells (An Essay upon single vision with two eyes, by W. C. Wells, M.D., London, 1818), the former to designate a horizontal line connecting the optic axes at their points of emergence from the cornea, and the latter a line passing forward from the middle of the visual base through the point of mutual intersection of the optic axes.

in the common axis PCB A, at short distances apart, and about ten inches from the face, and the optic axes I fn B and igo B then brought to bear upon the central body at B, whilst the visual angles Ben and B d C, or Be A and B d A, are made to include either the more or the less distant one at A or C, the body at the point of intersection of the optic axes in B will be seen single, whilst the included body at A or C will appear double. If now the axial and included bodies be slowly approximated, the two images of the latter will be observed to gradually approach each other, till ultimately they are converted into one, when the bodies have attained a certain close proximity.

If, on the contrary, the bodies be separated to a greater dis tance from one another, the two images of the included body will be likewise separated in a corresponding ratio till they are entirely lost sight of. If, whilst the bodies occupy their original position, one eye be suddenly closed, we lose sight of one of the images of the included body, the one lost being that of the

op

posite side, when the body which yields it is the proximal one, and that of the same side when that body is the distal one.

In this experiment, the object situated at the intersection of the optic axes produces but a single visual perception, because the image of it is depicted on the retina of each eye within the circle of visual unity at b and i, whilst the included body gives rise to a double perception, because the rays of light proceeding from it are incident upon the retina at j and k or h and m, outside that circle, and within the region of visual duality. When, however, the included is made slowly to approach the axial body, it will arrive at a point ere it come into contact with the latter, at which it produces but a single perception, because, during this movement, the retinal angle, decreasing in an equal ratio with the visual angle, will come finally to be less than 3°, and therefore to fall within the circle of visual unity. The abolition of one of the two images of the included body, consequent upon the closing of either eye, is due to the fact that its apparent position is advanced or retracted, as the case may be, towards the visual curve described by the optic axis as a radius. When the body included in the visual angle is the proximal one at C, its images are projected to opposite sides of the common axis PC BA, towards the visual curves described by the radii i B and l B, but encountering in their course thither the axial rays B nfl and Bog i proceeding from the object at B, they are arrested at n and o, where the lines of their visible direction intersect those rays. Hence, the closure of the right eye is attended with the loss of the left image at n, and of the left eye with that of the right image at o. When, on the contrary, the included body is the distal one at A, its images are retracted quite to the visual curves described by the radii i B and 1 B, because the lines of their visible direction meet no interruption from the axial rays Bnfi and Bog i; and since the altered position of the image is on the same side of the common axis as the eye which takes cognizance of it, the closure of the right eve in this case involves a loss of the right image, and that of the left eye a loss of the left image. The apparent change of position of the included object in this experiment, is probably the result of ocular adjustment; it is impossible that the eye can adapt itself at one and the same time to the vision of two objects placed at different distances from it; and as only one of these can be in the optic axes, and the length of the axial ray determines the adjustment of the eye, the apparent position of the other is altered by the actual state of adjustment, for the purpose, as it would seem, of equalizing its distance to the length of the axial ray.

Third experiment.-If we look through two small tubes so

held before the eyes that the optic axes shall traverse them as nearly as possible in parallel lines, we see two luminous circles of exit. If, now, the tubes and axes be made slowly to converge towards a common point in front, we come at length to see but a single circle. If, over each orifice a slip of coloured glass be placed as in the experiment of Dutours, one being yellow and the other blue, a single luminous circle will be seen as before; but this will not be of the intermediate colour, green, the two colours being superposed, not blended, and a portion of each visible at the same time. In the first part of this experiment two luminous discs are seen, because the optic axes normally converge in front, and under no circumstances, as I believe, diverge, or even assume the mutual relation of parallel lines, in the healthy eye: hence the rays of light admitted through divergent or parallel tubes intersect the axes at angles more or less acute, and if the latter exceed 04 and are less than .533 inch, the rays fall on the retina within the region of visual duality, and give rise to a double perception of the orifice. With reference to the non-fusion of the colours, the explanation I am disposed to offer is, that the course of the optic axes being interrupted by the coloured glass before reaching the point of mutual intersection, the colour placed in each makes an unmixed impression on the corresponding retina, and this impression being uninterrupted preserves its individuality, although the apparent situation of the glass which produces it is projected to the "plane of the horopter", i.e., a plane at right angles to that of the optic axes, and passing through the point of their mutual intersection; here the coloured discs are accordingly seen to overlap without being blended. In the familiar experiment of rapidly rotating before the eyes a disc painted in the seven colours of the solar spectrum, the compound white is produced, because the impression made by each primitive colour, though transitory, is of sufficient duration to last till those of the complemental colours are superadded.

The advantages conferred by the use of two eyes in viewing objects, conjoined with the faculty of forming a single conception from a double image, are:

Firstly, as shown by Jurin,' objects seen with both eyes appear brighter than when viewed with one, by about one-thirteenth part.

Secondly, the individual is thereby enabled to judge of distance through the muscular sense exercised in the movements of the eyes, whilst directing their axes towards the object to be viewed, as observed by Sir C. Bell; for, although in monocular vision the

7 Smith's Optics, vol. ii. p. 107, et sequent. 8 Philosophical Transactions, 1823, p. 178.

operation of directing the axis of one eye to near and distant objects requires an equal degree of muscular contraction, and a proportionate exercise of the muscular sense in the muscles brought into action, as when both eyes are used, still an equally certain means is not thereby afforded by which to judge of distance; because, whilst a single eye is being used in viewing an object placed in the common axis, a change in the position of the object towards the right or left side would require a contraction of the rectus muscle of the same side, for the purpose of altering the direction of the axis, differing only in degree from that necessary when the object is brought closer to the eye, or removed to a greater distance from it; and hence a lateral change of position in the object might be readily mistaken for one in the line of the common axis. This source of error is obviated by the use of both eyes, which enables us to distinguish between the changes of position indicated; for if the object move from the mesial line towards either side, the external rectus muscle of one eye and the internal rectus of the other are called into consentaneous action, whilst a movement towards, or from the face, in the direction of the common axis, would be followed by a contraction of the internal or external recti of both eyes, as the case might be.

It is worthy of remark, as bearing on this subject, that the internal and external recti are supplied with nerve-filaments from distinct sources, namely, the third and sixth cerebral nerves. If it be admitted that the muscular sense is an endowment conferred upon muscle by its nerve of supply, the exercise of that sense through distinct nervous channels, in the two cases supposed in the last paragraph, will obviously serve as a means of distinguishing between them. As the distance of the object from the eye must regulate the angle formed by the visual axes convergent upon it, the intuitive knowledge which we possess of the magnitude of this angle, by what Porterfield aptly terms "the natural geometry of the eyes", must serve as a means of approximately determining distance. When, however, but one eye is made use of, we have only two sides of the triangle, namely, one axis and the visual base, by which to measure the angle under which the object is viewed, which, being insufficient for that purpose, we necessarily fail in the attempt to determine distance by this means. Inasmuch, however, as the angle in question diminishes less sensibly for very distant objects, we are unable to determine with equal accuracy the distance, and consequently the magnitude, of objects so situated.

Thirdly, the capacity of forming a single conception from a double image confers the further advantage of enabling the