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of the spot rests upon the entrance of the optic nerve, it is not perceived, and hence this region of the retina is called the blind spot.
6. The impression made by light upon the retina not only remains during the whole period of the direct action of the light, but has a certain duration of its own, however short the time during which the light itself lasts. A flash of lightning is, practically, instantaneous, but the
A. Branched pigment cells from the deep layer.
B. Pigment epithelium. a, seen in face; b, seen in profile; c, pigment
sensation of light produced by that flash endures for an appreciable period. It is found, in fact, that a luminous impression lasts for about one-eighth of a second; whence it follows, that if any two luminous impressions are separated by a less interval, they are not distinguished from one another.
For this reason a 66 Catherine-wheel," or a lighted stick turned round very rapidly by the hand, appears as a circle of fire; and the spokes of a coach wheel at speed are not separately visible, but only appear as a sort of opacity, or film, within the tire of the wheel.
7. The excitability of the retina is readily exhausted. Thus, looking at a bright light rapidly renders the part of the retina on which the light falls, insensible; and on looking from the bright light towards a moderately-lighted surface, a dark spot, arising from a temporary blindness
of the retina in this part, appears in the field of view. If the bright light be of one colour, the part of the retina on which it falls becomes insensible to rays of that colour, but not to the other rays of the spectrum. This is the explanation of the appearance of what are called complementary colours. For example, if a bright red wafer be stuck upon a sheet of white paper, and steadily looked at for some time with one eye, when the eye is turned aside to the white paper a greenish spot will appear, of about the size and shape of the wafer. The red image has, in fact, fatigued the part of the retina on which it fell for red light, but has left it sensitive to the remaining coloured rays of which white light is composed. But we know that if from the variously coloured rays which make up the spectrum of white light we take away all the red rays, the remaining rays together make up a sort of green. So that, when white light falls upon this part, the red rays in the white light having no effect, the result of the operation of the others is a greenish hue. If the wafer be green, the complementary image, as it is called, is red.
8. In some persons, the retina appears to be affected in one and the same way by rays of light of various colours, or even of all colours. Such colour-blind persons are unable to distinguish between the leaves of a cherry-tree and its fruit by the colour of the two, and see no difference between blue and yellow cloth.
This peculiarity is simply unfortunate for most people, but it may be dangerous if unknowingly possessed by railway guards or sailors. It probably arises either from a defect in the retina, which renders that organ unable to respond to different kinds of luminous vibrations, and consequently insensible to red rays or yellow rays, &c., as the case may be, or it may proceed from some unusual absorptive power of the humours of the eye which prevents particular rays from reaching the retina; or the fault may lie in the brain itself.
9. The sensation of light may be excited by other causes than the impact of the vibrations of the luminiferous ether upon the retina, Thus, an electric shock sent through the eye, gives rise to the appearance of a flash of light and pressure on any part of the retina produces a luminous image, which lasts as long as the
middle, provided with circular and radiating unstriped muscular fibres, and capable of having its central aperture enlarged or diminished by the action of these fibres, the contraction of which, unlike that of other unstriped muscular fibres, is extremely rapid. The edges of the iris are firmly connected with the capsule of the eye, at the junction of the cornea and sclerotic, by the connective tissue which enters into the composition of the so-called ciliary ligament. Unstriped muscular fibres, having the same attachment in front, spread backwards on to the outer surface of the choroid, constituting the ciliary muscle (Fig. 75, 4). If these fibres contract, it is obvious that they will pull the choroid forwards; and as the frame, or suspensory ligament of the lens, is connected with the
FIG. 76.-VIEW OF FRONT HALF OF THE EYEBALL SEEN FROM BEHIND. a, circular fibres; b, radiating fibres of the iris; c, ciliary processes; d, choroid. The crystalline lens has been removed.
ciliary processes (which simply form the anterior termination of the choroid), this pulling forward of the choroid comes to the same thing as a relaxation of the tension of that suspensory ligament, which, as I have just said, like the lens itself, is highly elastic.
The iris does not hang down perpendicularly into the space between the front face of the crystalline lens and the posterior surface of the cornea, which is filled by
the aqueous humour, but applies itself very closely to the anterior face of the lens, so that hardly any interval is left between the two (Figs. 75 and 77).
The retina, as we have seen, lines the interior of the eye, being placed between the choroid and vitreous humour, its rods and cones being imbedded in the former, and its anterior limiting membrane touching the latter.
About a third of the distance back from the front of the eye the retina seems to end in a wavy border called the ora serrata (Fig. 75, 9), and in reality the nervous clements of the retina do end here, having become considerably reduced before this line is reached. Some of the connective tissue elements however pass on as a delicate kind of membrane at the back of the ciliary processes towards the crystalline lens.
18. The eyeball, the most important constituents of which have now been described, is, in principle, a camera of the kind described above--a water camera. That is to say, the sclerotic answers to the box, the cornea to the watch-glass, the aqueous and vitreous humours to the water filling the box, the crystalline to the glass lens, the introduction of which was imagined. The back of the box corresponds with the retina.
But further, in an ordinary camera obscura, it is found desirable to have what is termed a diaphragm (that is, an opaque plate with a hole in its centre) in the path of the rays, for the purpose of moderating the light and cutting off the marginal rays which, owing to certain optical properties of spheroidal surfaces, give rise to defects in the image formed at the focus.
In the eye, the place of this diaphragm is taken by the iris, which has the peculiar advantage of being self-regulating dilating its aperture, and admitting more light when the light is weak; but contracting its aperture and admitting less light when the illumination is strong.
19. In the water camera, constructed according to the description given above, there is the defect that no provision exists for adjusting the focus to the varying distances of objects. If the box were so made that its back, on which the image is supposed to be thrown, received distinct images of very distant objects, all near ones would be
indistinct. And if, on the other hand, it were fitted to receive the image of near objects, at a given distance, those of either nearer, or more distant, bodies would be blurred and indistinct. In the ordinary camera this diffi-culty is overcome by sliding the lenses in and out, a process which is not compatible with the construction of our water camera. But there is clearly one way among many, in which this adjustment might be effected-namely, by changing the glass lens; putting in a less convex one when more distant objects had to be pictured, and a more convex one when the images of nearer objects were to be thrown upon the back of the box.
But it would come to the same thing, and be much more convenient, if, without changing the lens, cne and the same lens could be made to alter its convexity. This is what actually is done in the adjustment of the eye to distances.
20. The simplest way of experimenting on the adjustment of the eye is to stick two stout needles upright into a straight piece of wood, not exactly, but nearly in the same straight line, so that, on applying the eye to one end of the piece of wood, one needle (a) shall be seen about six inches off, and the other (b) just on one side of it at twelve inches' distance.
If the observer look at the needle b, he will find that he sees it very distinctly, and without the least sense of effort; but the image of a is blurred and more or less double. Now let him try to make this blurred image of the needle a distinct. He will find he can do so readily enough, but that the act is accompanied by a sense of effort somewhere in the eye. And in proportion as a becomes distinct, b will become blurred. Nor will any effort enable him to see a and b distinctly at the same time.
21. Multitudes of explanations have been given of this remarkable power of adjustment, but it is only within the last few years that the problem has been solved, by the accurate determination of the nature of the changes in the eye which accompany the act. When the flame of a taper is held near, and a little on one side of, a person's eye, anyone looking into the eye from a proper point of view, will see three images of the flame, two upright and