sition of the eyes, and the faculty of single vision with a double organ equally characteristic of these two orders, and may not unreasonably be supposed to be inseparably related to, and indispensable to the existence of, this faculty, according to the rule in physiology of inferring functional from structural peculiarity. Whether anything resembling the yellow spot exists in the eyes of nocturnal birds I have not been able to determine, not having had an opportunity of examining the organ in one of these animals whilst engaged in preparing the present article.

From the arguments now stated, supported by experiments to be detailed further on, I have been led to the conclusion that unity of visual perception with two eyes is inseparably associated with the yellow spot of Sömmering, and may invariably be inferred from its presence, and that this property is limited to a circle, having as its centre the foramen centrale, and a radius of about 04 inch. This circle I propose to designate as the "region of visual unity", within which the rays of light proceeding from an object must fall, in order to produce a single perception from a binocular impression. Outside this circle, and concentric with it, is another, having a radius of 0-533 inch:the latter I would name the "region of visual duality", because I believe that the image of an object depicted within it, and outside the region of unity in both eyes simultaneously, produces a double perception in the sensorium. That portion of the retina extending from the region of duality to the ora serrata or free border, is probably insensible to luminous impressions, and serves some other purpose in the economy of the eye, with which we are still unacquainted. Diagram fig. 4 (p. 483), modified from Valentin, will render the preceding observations more intelligible. It represents a section of a graduated sphere, which may be assumed, for the purpose of illustration, to exhibit a magnified view of the eye.

First experiment.-If the optic axis ki be directed to an object situated at 90°, the object will be depicted on the yellow spot at l, and will be seen with its maximum of distinctness. If now the eye be slowly rotated upon its centre, k, to either side, the object, being stationary, will continue to be seen with undiminished distinctness till the axis pass 87° or 93°, as the case may be. During this rotatory movement of the eye, the image of the object travels pari passu in the same direction (or rather, the image remains stationary whilst the retina moves), from 1 to o or I to p, through the retinal angle ko or l k p; i.e., over a space measured by the radius of the circle of "visual unity". If the eye still continue to rotate in the same direction, the perception of the object grows gradually less distinct till the axis reach 50° or

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 p kn 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)

с

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 / 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 distance 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 7 B, because the lines of their visible direction meet no interruption from the axial rays Bnfi and Bog i; ad 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 eye 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.