ADAPTATION OF THE EYE TO DISTANCE.

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It must, however, be observed that perfect discrimination resides only in a limited spot of the eye, or in that part where an object lies when we concentrate our attention upon it. Thus, although I see a wide prospect, my power of minute discrimination is confined to the place where I am said to be looking, that is, on the line of the axis of the eye. In the borders of the field of view everything is dim and vague.

4. On the Adaptation of the Eye to Vision at different Distances. If I see an object distinctly six inches distant from the eye, all objects at a greater distance are indistinct. The image of the near object falls correctly on the retina, the images of remote objects are formed in front of the retina. By a voluntary effort I can adapt the eye to see a far off object with tolerable clearness, but it then happens that any near body becomes confused. The question arises what is the change produced upon the eyeball in the course of this adaptation from near to far, and from far to near, and what apparatus effects the change. Many answers have been given to this question, but no one is yet completely established. The following remarks are all that are admissible in this place.

(1.) The change, whatever it be, seems not to depend upon the external muscles of the eye. It has been conceived that the recti and oblique muscles might by being strongly exerted alter the shape of the globe, and with that its focal distance, but such idea does not now obtain credence. On the other hand, the convergence of the two eyes is exclusively effected by these muscles, and along with this convergence the internal adjustment of each eye is found to take place, implying some fixed association between the movements of convergence or divergence and the alteration of the ball.

(2.) The ciliary muscle already alluded to, from its position and attachments, would draw the crystalline lens nearer to the cornea, and thus alter, it is believed, the focal distance of the lens. In the effort to view near objects this muscle would be contracted; in the case of more remote objects it would be relaxed, and the eye would have to recover its shape by mean of the elasticity of the parts. It is interesting to notice the

this adjusting faculty of the eye is greatly impaired or altogether lost by extraction of the lens, or by paralysing the ciliary and iridial muscles by belladonna.'

(3.) The movements of the iris itself have some connexion with the distance of the objects. The pupil is found to contract during near vision, and to expand in looking at remote objects. This is believed to be a coincidence with the converging and diverging action of the two eyes; but there is no sufficient proof that the eye is adjusted by this circumstance. It is convenient in remote vision to have the pupil expanded for the sake of light, and in near vision to contract it, in order to exclude a portion of the extreme rays, and thereby obtain a more distinct image.

(4) The eyeball is subject to alteration chiefly for near distances. Between the smallest visible distance, say four inches, and three feet, nearly the whole range of the adjustment is gone through. When we compare distant objects of varying remoteness, as, for example, thirty feet with one hundred, or a thousand, very little change is effected on the form of the eyeball, the adjustment then depending on the greater or less convergence of the two eyes. This leads us to the subject of double vision.

5. Of single Vision with two eyes. Binocular Vision.— Among the questions long discussed in connexion with sight, was included the inquiry, why with two eyes do we see objects. single? Answers more or less satisfactory were attempted to be given; but since the year 1838, an entirely new turn has been given to the discussion. In that year, Professor Wheatstone gave to the Royal Society his paper on Binocular Vision, wherein he described his 'stereoscope,' or instrument for imitating and illustrating the action of the two eyes in producing single vision. The following quotation is from the opening paragraph:

'When an object is viewed at so great a distance that the optic axes of both eyes are sensibly parallel when directed towards it, the perspective projections of each, seen by each eye separately, are similar, and the appearance of the two eyes is precisely the same as when the object is seen by one

eye only. There is in such case no difference between the visual appearance of an object in relief, and its perspective projection on a plane surface; and hence pictorial representations of distant objects, when those circumstances which would prevent or disturb the illusion are carefully excluded, may be rendered such perfect resemblances of the objects they are intended to represent as to be mistaken for them; the Diorama is an instance of this. But this similarity no longer exists when the object is placed so near the eyes that to view it the optic axes* must converge, and these perspectives are more dissimilar as the convergence of the optic axes becomes greater. This fact may be easily verified by placing any figure of three dimensions-an outline cube, for instance,-at a moderate distance before the eyes, and while the head is kept perfectly steady, viewing it with each eye successively while the other is closed. The figure represents the two perspective projec

FIG. 9

d

tions of a cube; a is seen by the right eye, and d is the view presented to the left eye, the figure being supposed to be placed about seven inches immediately before the spectator.'

'It will now be obvious why it is impossible for the artist to give a faithful representation of any near solid object, that is to produce a painting which shall not be distinguished in the mind from the object itself. When the painting and the object are seen with both eyes, in the case of the painting two

The optic axis of the eye is the line of visible direction for distinct vision, or a line proceeding from the central point of the retina, and passing through the centres of the lenses of the eye.

similar pictures are projected on the retinæ, in the case of the solid object the pictures are dissimilar; there is therefore an essential difference between the impressions on the organs of sensation in the two cases, and consequently between the perceptions formed in the mind; the painting, therefore, cannot be confounded with the solid object.'

Mr. Wheatstone then goes on to describe his stereoscope, or apparatus for imitating the effect of double vision. This instrument is now so common that I need not insert a description here. Two forms of it are in use, but the principle of both is the same; that is, to present two different pictures of an object, such as the cubes above figured, one to each eye exclusively, and so as that they may appear only one picture. By so doing, a vivid appearance of solid effect starts forth to the view. While the two pictures are seen apart from each other they are looked upon simply as pictures, but when they are made as it were to cover one another in the field of view, there is no longer a picture but a solid reality. The effect must be experienced in order to be appreciated. It is impossible to resist the conclusion that our perception of solidity or of three dimensions, is most powerfully aided by the concentration of the two eyes upon the same object, and by the view thus derived through the two dissimilar pictures.

The drawing of figures to suit the stereoscope was a troublesome and tedious operation. But no sooner was Daguerre's discovery announced than Professor Wheatstone laid hold of it to furnish him with his binocular views; and since then the use of the stereoscope has been vastly extended. It is only necessary to daguerreotype two views of the same object corresponding to the views of the two eyes, that is, to turn it slightly round for the second view; the two pictures placed in the stereoscope give the perfect appearance of relief.

When the two different pictures seem to cover one another in the field of view, the singleness of vision takes place along with the effect of solidity above mentioned. This fact is supposed to imply a certain correspondence of the two eyes, or the existence of corresponding points or places in each, such that when binocular pictures fall on any pair, one single effect

arises in the brain. must exist, but this fact.

SENSATION OF LIGHT.

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Doubtless some general correspondence cannot be maintained as an invariable

There are still difficulties in the way of the full explanation of double vision, and of the perception of solidity arising from it. What remains to be said upon it in the present chapter will fall properly under the last head of the subject, the Sensations of sight.

6. Before quitting the consideration of the Eye, I should mention that the seeing of objects erect by means of an inverted image on the retina has been conceived as a phenomenon demanding explanation. But to make this a question at all is to misapprehend entirely the process of visual perception. An object seems to us to be up or down according as we raise or lower the pupil of the eye in order to see it; the very notion of up and down is derived from our feelings of movement, and not at all from the optical image formed on the back of the eye. Wherever this image was formed, and however it lay, we should consider that to be the top of the object which we had to raise our eyes or our body to reach.

7. And now as to the Sensations, or the proper mental elements of Sight. The feelings arising from Sight alone make only one class of these sensations; the combination of the optical and muscular states gives birth to the most various and interesting department of feelings connected with vision.

8. I shall commence with the sensation of mere light, and shall take the diffused solar radiance as the leading example. This is one of the most powerful of the simple influences that affect the human sense. Light is eminently a source of pleasure, which rises in degree, within certain limits, in proportion to the abundance of the luminous emanation. In clear strong sunshine, filling the entire breadth of the eye and freshly encountered, there is a massive powerful influence of pleasurable elation, acutely felt. The effect is sufficiently intense, massive, and keen to make a predominating or engrossing emotion, like alimentation, warmth, or the other powerful forms of organic and muscular feeling formerly described. The quality that most distinguishes it from these