" Let MN, for example, be an object at a considerable distance from the eye, EF O. Rays of light diverging from the points M, N, will be converged by the refraction of the humours to points m, n, upon the retina, where they will form an inverted image of it, in the same manner as an image is formed in a camera obscure. That such an image is actually formed on the back of the eye may be easily proved by paring away the sclerotic coat of the eye of an ox, with a sharp knife, till it is sufficiently thin to allow the image to be seen through it.
" In what manner the retina, thus impressed with a distinct image of an external object, conveys to the mind, through the niedium of the optic nerve, of which it is the expanded termination, a .Uow ledge of the existence, the position, and the magnitude of that object, is not known, and probably never will be. Certain facts, however, or laws of vision, have been deduced from observation, and merit our attentive consideration.
" 1. On the direction of visible objects.—When the mind sees the extremity M of any object M N, by means of rays flowing from M and collected at m, the retina receives these rays at different degrees of obliquity, and yet the point M is seen only in one direction, namely, in the direction of the central ray of the cone whose apex is at m. This however does not wise from the ray being the result ant, as it were, or the mean of the directions of all the other rays ; for if we dose up all the pupil excepting a small opening at its margin, the point M will be represented at m only by the most oblique rays of the conical pencil, and yet it will still be seen in the same direction as before. Hencewe conclude, that when a ray of light falls upon any point m of the retina, in any direction, however oblique to its surface, the object will be seen in the direction of a line perpendicular to the retina at the point m. As the surface of the retina is a portion of a sphere, these perpendiculars must all pass through one point, which may be called the centre of visible direc tion; because every point of an external object will be seen in the direction of a line joining that centre and the given point. The truth of this we have established by marking the perfect stability of the image of any object, when it is seen by different points of the retina when the eyeball alone is moved. Hence the centre of visible direction is a fixed point in the vitreous humour ; and as it never changes its place during the rotation of the eyeball, it must be coincident with the centre round which that rotation is performed. In consequence of this coincidence, and in virtue of the law of visible direction, an arrangement of consummate skill, the great Author of nature has provided for the perfect stability of every point in the images of external objects.
" 2. Cause of erect vision.—As the humours of the eye act exactly like a convex lens of an equivalent focal length, an inverted picture of external objects will, for the reasons already assigned be formed upon the retina. Many philosophers of eminence have perplexed themselves very unnecessarily, in attempting to deduce erect vision from inverted images. The law of visible direction removes at once every difficulty ; for as the lines of visible direction must necessarily cross e,ach other at the centre of visible direction, those from the lower part of the image must go to the upper part of the object, and those from the upper part of the image go to the lower part of the object, and hence an erect object is the necessary result of an inverted image.
. " 3. Distinct and indistinct vision in the same object.—When we look intensely at any point of an object in order to examine it with care and attention, we direct to that point the axis of the eye, and consequently, the image of that point falls upon the central hole in the retina. Every other point of the same object is seen indistinctly, and the indistinctness increases with the distance of the point from that which is seen distinctly. The only perfectly distinct point of vision, therefore, is that where there is no retina; but we are not entitled to ascribe this to the absence of the nervous matter, as the gradual increase of distinctness towards the central hole does not appear to be accompanied with a gradual dimunition in the thickness of the retina.
" 4. Indistinctness of vision at the base of the optic nerve.—It was discovered by M. Mariotte, that when the image of any object fell upon the base of the optic nerve, the object disappeared. In order to prove this experimentally, fix on the side of a room, and at the height of the eye, three wafers, two feet distant. Stand opposite to the middle wafer with one eye shut, and, beginning near the wall, retire gradually from it, (looking always at the outside wafer which is on the same hand as the covered eye,) till the middle wafer disappears. This will be found to take place at about five times the distance at which the wafers are placed, and when it does happen, the other wafers will be plainly seen. If we use canales in place of wafers the middle one will not disappear, hut it will become a cloudy mass of light. The base of the optic nerve, therefore, is not insensible to light, it is only unfit for giving distinct vision of those objects whose images fall upon it. M. Le Cat considered the size of this portion of the retina to be about one-third or one-fourth of a line ; hut Daniel Bernoulli found it to be about one-seventh part of the diameter of the eye." The foregoing explanation is clear and accurate, with the excep tion of some remarks on the direction of visible objects, in para graph 1. These it is important for us to correct. When a pencil of light from a luminous point M enters the eye and is brought to a focus at m upon the retina, the line of visible direction is not the axis of the cone of rays within the vitreous humour of the eye, but a line drawn perpendicular to the retina at the point m, and which passes through the centre of the eyeball, or, which is the same thing, through the centre of visible direction, as correctly stated in latter part of the paragraph referred to in terms which evidently tradict the former part of the same paragraph. If the reader i draw a figure showing the course of an oblique pencil through eye, he will perceive that the axis of the final cone of rays within vitreous humour is a line joining the point m, with a point some [ere near the pupil of the eye, while the line of visible direction a line joining m with a point near the centre of the eyeball. lien the retina receives a blow at m, no matter how obliquely, direction of the supposed force is referred by the mind to a line rpendicular to the retina at m.
Binocular Vision.
When both eyes are employed simultaneously vision is said to he ' Binocular." The principles of it are easily explained, and will be tnderstood by means of the following figure, and a few words of Let L, R, be the two eyes employed simultaneously. Then only one point can be seen distinctly at any instant of time, and that is the point to which both the optic axes are directed and at which they meet, as shown by the point A. in the figure. If B, C, be other points, then the mind is simply warned of their existence by their images upon the sides of the retinae, and distinct vision of them is not obtained. But the eyeballs are capable of being turned in their sockets with extreme ra pidity and precision, and the optic axes may be united at several points in suc cession by muscular motions which are almost instantaneous, so that it is by the comparison which the mind is able to form of the efforts made by the voluntary muscles in enlarging or contracting the angles LA R, LBR, LC R, that the most certain estimate of the comparative dis tance of near objects is obtained. Hence arises the important difference between monocular and binocular vision, for by means of the latter additional certainty is given to dur appreciation of distance, and hence arise all the remarkable phenomena of stereoscopic effect.
As a proof that only one object is seen distinctly at a time, it is only necessary to hold up the forefinger at a distance of a foot from the nose, and look intently at some object several yards bevond it ; the finger will then appear double and transparent, distani objects being seen through it indistinctly.
No idea of the distance of an object can be obtained from the changes which take place in the focussing arrangements of a single eye to suit the different distances of objects between the limits of a few inches and infinity.