Distinct vision. — Dugald Stewart, in his " Philosophy of the Human Mind," proposes this question : Suppose the eye to be fixed in a particular position, and the picture of an object to be painted on the retina, does the mind perceive the complete figure of the ob ject at once, or is this perception the result of the various perceptions we have of the dif ferent points in the outline ? He arrives at the conclusion that the mind does at one and the same time perceive every point in the outline of the object, for perception, like con sciousness, is an involuntary operation as no two points, however, of the outline are in the same direction, every point by itself constitutes a distinct object of attention to the mind ; but these acts of attention are performed with such rapidity, that the effect is the same as if the perception were instantaneous. When the eye is directed to any point of a land scape, it secs with perfect distinctness only that image of it which is directly in the axis of the eye ; but the extreme mobility of the eye, together with the duration of the impres sions upon the retina, enable us to take in every part of the view with equal distinctness. In all probability, it is only in the axis of the eye, corresponding with the yellow spot, that vision is perfect ; the posterior part of the retina is certainly better adapted to receive images than the anterior, where the grey nervous layer becomes thinner and thinner towards the border. Dr. Young calculated that the range of motion in the eye-ball is 55° in every direction, so that the head being fixed, a single eye may have perfect vision of any point within a range of 110°. He fur ther ascertained, by fixing the eye in the most natural direction, namely, forwards, and a little downwards, and by then moving a luminous object before it in various direc tions, that the range of vision upwards was 60°, downwards 70°, inwards 60°, and out wards 90°, giving an entire horizontal play of 150°, and a total vertical play of The small portion of retina corresponding to the extremity of the optic nerve, is insen sible to visual impressions. Volkinann states that he has satisfied himself by calculation that the small insensible spot corresponds exactly with the dimensions of the central artery. Dr. Young determined that the dis tance of the centre of the optic nerve from the visual axis is of an inch, and that the diameter of the most insensible part of the retina is of an inch. It is to be borne in mind that the fibrous lamina of the grey nervous layer of the retina is at this point evolving itself from the nerve, and is not yet invested with the vesicular or other laminae. Mr. Bowman has well remarked that this in capacity of vision at the entrance of the optic nerve seems to be essential to the mode of junction of the retina with the nerve, since it appears to have been the chief reason why the nerve was not made to enter in the axis of the eye. If the blind spot had been situated in the axis, a blank space would have always existed in the centre of the field of vision, since the axis of the eyes in vision are made to correspond. But as it is, the blind spots do not correspond when the eyes are directed to the same object ; and hence the blank which one eye would present, is filled up by the opposite eye. Mariotte, was the first who described the existence of these blind spots, and they may be discovered by the following simple experiment — Let two black dots be placed one inch apart on a sheet of white paper : if the left eye be closed, and the dots are regarded at the usual distance for distinct vision, the attention, however, being particu larly directed to that on the right hand, the other dot will be found to disappear the moment the pencil of rays proceeding from it falls upon the centre of the entrance of the optic nerve.
It has been already stated, that when the rays from an object meet in foci on the retina, a distinct image is formed : should the focus, however, be before or beyond the retina, it is evident that some indistinctness must be caused, for each point of the retina would then receive rays from several points of the object. It is to be borne in mind, that the nearer the object to the eye, the greater is the divergency of the incident rays; and the greater their divergency, the more distant is their focus. When the retina corresponds,
or nearly corresponds, to the points of con vergence of the several pencils of light, dis tinct vision is obtained, the usual distance being from eight to ten inches, at which dis tance, reading or writing is naturally per formed. If, when writing, for instance, we removed the head further, or approached it nearer, and no alteration took place in the eye, vision would become indistinct, because the focus would be altered, and would either fall short of, or be thrown beyond, the re tina : should the rays, however, very nearly unite upon the retina, vision, of large ob jects especially, may prove sufficiently dis tinct, though not perfectly clear. A just distinction was, therefore, drawn by Junin between perfect vision and distinct vision, the perfection of vision depending on the distance alone of objects, whilst their distinctness would be regulated not less by'their size, than by their distance from the eye.
The pupil of the eye performs an important part in enabling us to see objects distinctly. When looking intently at a near object, the pupil contracts, thereby limiting the rays which pass through it to the most central, and stopping the progress of those more di vergent rays which, not converging to foci on the retina, would cause circles of dissipation ; upon the same principle, vision of near objects is assisted by a pin-hole aperture. Let a per son thrust a pin through a card, and make a clean hole ; then let him hold a book so close to his eyes that the type becomes con fused ; if he now looks through the pin-hole, he will again see it distinctly, because the card answers the same purpose as a further contraction of the pupil, arresting the progress of those rays which are too divergent, and limiting those entering the eye to the central rays, which, from their trifling divergence, unite correctly on the retina. The same ad vantage is gained by a near-sighted, and by a presbyopic eye. In the one case vision is im proved by the card stopping the rays, which would converge to foci in front of the retina ; in the other, by its arresting those which tend to foci behind the retina. So that this simple experiment frequently makes a difference of several inches in the vision of myopic and presbyopic persons.
A curious experiment devised by Father Scheiner, has reference to this point. If we make in a card two small orifices with a needle, at a less distance from each other than the diameter of the pupil, and hold these openings close to the eye, a double image of a small object held within the visual distance, — a pin's head, for instance— will be seen. From the pin's head there pass two very minute pencils of rays through the apertures into the eye. These rays converge towards a point lying behind the retina, and fall upon the retina at two different points. These are two isolated points of the circle of dispersion, which would exist upon the retina, if the other rays were not intercepted by the card. If we now withdraw the pin's head more and more, the images will approach, because the rays, falling upon the eye through the apertures, will diverge less, and will conse quently be refracted towards a point lying nearer to the retina. If the object be re moved from the eye to the distance of distinct vision, the two images will perfectly coincide, since all rays passing from one point, lying exactly at the distance of distinct vision, will be concentrated at one point of the retina. It may be asked, what are the conditions of adaptation necessary for an eye in looking through a fine aperture ? In its normal con dition, for the maintenance of which no effort is necessary, the eye is in the state necessary for seeing objects which lie at the distance of distinct vision. If a distant object be re garded through the small apertures, the rays passing through them into the eye must evi dently meet at one point before the retina, as the condition of each adaptation does not change in the eye: but the two pencils di verge again behind the point of intersection, striking the retina at two different points, when, consequently, distant objects will be seen double ; therefore, we only see a small object single, through two small apertures, when it lies at the distance of distinct vision.