determined by watching the image of a lamp flame reflected at the first surface of the lens: the image grows smaller when the eye is adapted for near vision. The small ciliary muscle seems unequal to these effects. More probably they are due to the constriction of the eyeball by two strong muscles called the " 'oblique," which gird the ball in a plane. at right angles to its axis. The contraction of this girth gives a slight elongation to the eyeball in the direction of its axis, i. e. increases the distance between the lens and the retina, and therefore fits the latter to receive the receding image of a nearer object. You can feel in your own eyes that in withdrawing their gaze from a distant object to an object very near you are performing a muscular exercise ; and in straining the eyes to the limits of near vision the muscular effort becomes very great, and may even be made decidedly painful; whereas in re-adapting the eyes to distant vision you feel that you are relaxing a muscle; you cannot strain your eyes for distant vision; the effort is only the effort of abstaining from muscular action, and giving the eyeball time to resume its shape. The limit of distinct near vision is about 6 inches, differing in different people. Some eyes have an undue convexity of cornea or lens, or an undue distance between lens and retina, so that parallel rays incident on the cornea are brought to a focus before they reach the retina. This defect is called "short sight," and cannot be remedied by any effort of the eye, though it is palliated by a contraction (instinctive, involuntary) of the pupil, or by almost complete closure of the eyelids, by which the marginal rays of the pencil are excluded, and the marginal rays are the ones that make confusion and indistinctness in the image. This accounts for the fact of a shortsighted person habitually compressing his eyelids till there is a mere chink between them. But the malformation is easily

remedied by the interposition of a double-concave or planoconcave eye-glass in front of the eye, which gives a slight divergence to the parallel rays from a distant object and throws the image of that object further back into the eye till it is within the range of the retina.”

there

Long-sight," or inability to see clearly objects that are within a foot or two feet, is due to loss of muscular power in the adjustment of the eye for near vision, and is a very common defect in old people. Also in advancing age is a flattening of the cornea, which throws images a little further back, and makes a greater effort required for bringing the retina into the focus. This is remedied by a convex eye-glass, such as old people generally use.

108. We give a figure shewing the formation of the image of an object of any size, only one point of which can of course lie upon the optical axis. The result is precisely as in a double convex lens, the centre of the eye, that is, the optical centre, lying at about the middle point of the pupil b.

Fig. 72.

Thus let POQ be an object and let C be the optical

centre of the compound lens. Then

Range of Vision.

109. When the eye looks straight forward, full face, its range of vision is much limited; above, by the projecting lid and eyebrow; below, by the projecting lid and cheekbone; inwards, by the projecting nose: but outwards there is no hindrance at all; the most oblique rays from the right meet with no obstacle to prevent their entrance into the right eye; and in like manner the left eye is open to the most oblique rays from the left. Exactly corresponding to this ordinary range of admission of rays from outer objects, is the range of sensitiveness possessed by the retina. This is shewn in figures (i) and (ii). The vertical section shews. the range of retinal sensitiveness above and below, and the horizontal section shews the range inward and outward. In the latter it will be noticed that the portion ih' of the retina of the right eye is much greater than the portion ih, i being the visual centre of the retina, in the axis of vision. in' receives rays from the right, unhindered: ih receives rays from the left, but the nose being in the way, it does not receive those of great obliquity, and there is therefore no need of retina for such rays. As nearly as I can estimate, the upward range is about 60°, the downward about 50o, the inward about 50°, the outward about 95°. Looking straight forwards, one can detect objects to right or left further back than the horizontal line joining the two eyes.

IIO.

Over the entrance of the optic nerve (Fig. 72 (ii)), there is a circular patch of the retina which is incapable of distinct vision, though not insensible to general impressions of light and colour. Looking with only one eye open, at very large print, it may be noticed that there is an obscuration of a batch of letters about 15° to the outer side of the centre of vision (to the left, looking with the left eye; to the

right, looking with the right eye). This patch of obscurity is about 5" in diameter. Looking with both eyes open, the blindness of each punctum cœcum is corrected by the sharp-sighted tract on the corresponding side of the other retina.

It is pretty well established that the left halves of the two retina are supplied with nerve fibres from the left half of the brain, and the right halves from the right. The evidence of this is got partly from dissection, and partly from observation of morbid phenomena: instances are recorded in which disease on one side of the brain was attended with permanent blindness of the corresponding sides of both retina. "Half-blindness" of a transient nature, affecting both eyes in the same way, points to the same conclusion. [Read an interesting paper by Dr Woollaston in the Philosophical Transactions of the Royal Society, 1824, I. p. 222.]

III. It will be observed that images on the retina are upside down, and we might therefore expect to see everything upside down. But since we see everything upside down, this effect is destroyed. "What has the mind to do with upside down so long as it gets evidence of position and motion of things external in harmony with the uprightness of the body, and the fact of gravity pulling everything to the earth ?"

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