Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/857

EYE 821 X Y obliquity of the incident ray. If, instead of one plate, we take two plates of equal thickness, one placed above the other, two images will be seen, and by turning the one plate with reference to the other, each image may be displaced a little to one side. The instrument consists of a small tele scope (fig. 14) T, the axis of which coincides with the plane separating the two glass plates C C and B B. When we look at an object X Y, and turn the plates till we see two objects xy, xtj touch- iug each other, the size of the image X Y will be equal to the distance the one object is displaced to the one side and the other object to the other side. Having thus measured the size of the reflection, it is not difficult, if we know the size of the object reflecting the light and its distance from the eye, to calculate the radius of the curved surface. (See Woinow s Opht/ialmomelrie, St Petersburg, 1871, and an account given in Appendix to M Kendrick s Outlines of Physiology, 1878.) The general result is that, in accommo dation for near objects, the middle reflected image becomes smaller, and the radius of curvature of the anterior surface of the lens becomes shorter. Fro. 14. Diagrammatic (5.) Absorption and Reflection of view of the Ophthalmo- Luminous Rays from the Eye. meter of Helmholtz. When light enters the eye, it is partly absorbed by the black pigment of the choroid and partly reflected. The reflected rays are returned through the pupil, not only follow ing the same direction as the rays entering the eye, but uniting to form an image at the same point in space as the luminous object. The pupil of an eye appears black to an observer, because the eye of the observer does not receive any of those reflected rays. If, however, we strongly illuminate the retina, and hold a lens in front of the eye, so as to bring the reflected rays to a focus nearer the eye, then a virtual and erect, or a real and reversed, image of the retina will be seen. Such is the principle of the ophthal moscope, invented by Helmholtz in 1851. Eyes deficient in pigment, as in albinos, appear luminous, reflecting light of a red or pink colour; but if we place in front of such an eye a card perforated by a round hole of the diameter of the pupil, the hole will appear quite dark, like the pupil of an ordinary eye. In many animals a portion of the fundus of th3 eyeball has no pigment, and presents an iridescent appearance. This is called a tapetum. It probably renders the eye more sensitive to light of feeble intensity. (6.) Functions of the Iris. The iris constitutes a diaphragm which regulates the amount of light entering the eyeball. The aperture in the centre, the pupil, may be dilated by contraction of a system of radiating fibres of involuntary muscle, or contracted by the action of another system of fibres, forming a sphincter, at the margin of the pupil. The radiating fibres are controlled by the sympathetic, while those of the circular set are excited by the third cranial nerve. The variations in diameter of the pupil are deter mined by the greater or less intensity of the light acting on the retina. A strong light causes contraction of the pupil; with light of less intensity, the pupil will dilate. In the human being, a strong light acting on one eye will often cause contraction of the pupil, not only in the eye affected, but in the other eye. These facts indicate that the phenomenon is of the nature of a reflex action, in which the fibres of the optic nerve act as sensory conductors to a centre in the encephalon, whence influences emanate which affect the pupil. It has been ascertained that if the fibres of the optic nerve be affected in any way, contraction of the pupil follows. The centre is probably in the anterior pair of the corpora quadrigemina, as destruction of these bodies causes immobility of the pupil. On the other hand, the dilating fibres are derived from the sympathetic ; and it has been shown that they come from the lower part of the cervical, and upper part of the dorsal, region of the cord. But the iris seems to be directly susceptible to the action of light. Thus, as was first pointed out by Brown-S6quard, the pupil of the eye of a dead animal will contract if exposed to light for several hours, whereas, if the eye on the opposite side be covered, its pupil will remain widely dilated, as at the moment of death. The pupil contracts under the influence (1) of an in creased intensity of light ; (2) of the effort of accommodation for near objects ; (3) of a strong convergence of the two eyes; and (4) of such active substances as nicotine, morphia, andphysostigmine; and it dilates under the influence (1) of a diminished intensity of light ; (2) of vision of distant objects ; (3) of a strong excitation of any sensory nerve ; (4) of dyspnoea ; and (5) of such substances as atropine and hyoscyamine. The chief function of the iris is to so moderate the amount of light entering the eye as to secure sharpness of definition of the retinal image. This it accomplishes by (1) diminishing the amount of light reflected from near objects, by cutting off the more divergent rays and admitting only those approaching a parallel direc tion, which, in a normal eye, are focussed on the retina ; and (2) preventing the error of spherical aberration by cutting oft divergent rays which would otherwise impinge near the margins of the lens, and would thus be brought to a focus in front of the retina. 3. SPECIFIC INFLUENCE OF LIGHT ON THE RETINA. The retina is the terminal organ of vision, and all the parts in front of it are merely optical arrangements for securing that an image will be accurately focussed upon it. The natural stimulus of the retina is light. It is often said that it may be excited by mechanical and electrical stimuli ; but such an observation really applies to the stimulation of the fibres of the optic nerve. It is well known that such stimuli applied to the optic nerve behind the eye produce always a luminous impression ; but there is no proof that the retina, strictly speaking, is similarly affected. Pressure or electrical currents may act on the eyeball, but in doing so they not only affect the retina, con sisting of its various layers and of Jacob s membrane, but also the fibres of the optic nerve. It is probable that the retina, by which is meant all the layers except those on its surface formed by the fibres of the optic nerve, is affected only by its specific kind of stimulus, light. This stimulus so ff aects the terminal apparatus as to set up actions which in turn stimulate the optic fibres. The next question naturally is, What is the specific action of light on the retina 1 Professors Holmgren of Upsala individually, and Dewar and M Kendrick conjointly, have shown that when light falls on the retina it excites a variation of the natural electrical current obtained from the eye by placing it on the cushions of a sensitive galvanometer. The general effect was that the impact of light caused an increase in the natural electrical current, during the continuance of light, the current diminished slowly, and fell in amount even below what it was before the impact, and that the with drawal of light was followed by a rebound, or second increase, after which the current fell in strength, as if the

eye suffered from fatigue.