EYE. 



129 



yellow margin called the foramen centrale. A mem 

 brune in the form of a flat ring, e e, divides the inte- 

 rior of the eye into two unequal portions, called the 

 anterior and posterior chambers. It is called the 

 uvea, or iris, and may be seen through the cornea 

 when looking to the front of the eye, being that 

 coloured ring between the white of the eye and the 

 dark spot in the centre. This membrane is differ- 

 ently coloured in diilerent individuals, being blue, 

 grey, or brown. The circular opening in the centre, 

 called the pupil, may, from the peculiar structure of 

 this membrane, be either increased or diminished 

 in size, in order to regulate the quantity of light 

 admitted ; as in weak light too few rays might be 

 admitted, and the objects be rendered indistinct, and 

 in a strong light too many might injure the organ. 

 This remarkable effect, which takes place with 

 great quickness, is thought to be produced by 

 means of two sets of muscular fibres, the one set 

 being circles surrounding the pupil, and the other 

 set crossing these, converging from the circumfer- 

 ence of the membrane towards the centre of the 

 pupil. The space d between the iris and cornea, t. e. , 

 the anterior chamber of the eye, is filled by a thin 

 transparent watery fluid, called the aqueous humour. 

 Immediately behind the iris is a transparent capsule 

 or bag, c c, filled by a clear fluid, and suspended by 

 the ciliary processes, g g. This is called the crys- 

 talline lens, immediately behind which is the vitreous 

 humour, V V, filling the greater portion of the eyeball. 

 Such is a general account of the anatomy of the 

 human eye, regarding its function as an organ of com- 

 munication between the mind and the external world ; 

 it is not our intention to consider it at any length 

 in the present article, but this curious and interesting 

 subject will be fully treated of under our articles 

 vision and optics. At present it will suffice to re- 

 mark, that the eye, considered as an optical instru- 

 ment, bears a strong resemblance to the camera ob- 

 scura (q. v.). The rays of- light proceeding from 

 any luminous object, and passing through the cor- 

 nea, are transmitted through the aqueous humour, 

 and, passing through the pupil, are received by the 

 crystalline humour, or lens, as it resembles a dou- 

 ble convex lens, and, collecting the rays, forms an 

 image of the external object on the retina, which is 

 an expansion of the optic nerve, and conveys the im- 

 pression to the brain. 



Blindness and defect of sight may arise from vari- 

 ous causes. When the transparency of the cornea, 

 the crystalline lens, or any of the humours, is destroyed, 

 either partially or entirely, then will partial or total 

 blindness follow, since no image can be formed upon 

 the retina : but, although alfthe humours and the 

 cornea be perfectly transparent, and retain their 

 proper forms, which is likewise necessary to distinct 

 vision ; yet, from weakness, or inactivity of the optic 

 nerve, weakness of sight or total blindness may en- 

 sue. To the first class belong those diseases called cat- 

 aract, ophthalmia, &c; and to the second, amaurosisor 

 gutta serena, &c. Although a person may not be under 

 the influence of any of these diseases, yet defective 

 vision may arise from the crystalline lens being so 

 convex as to form an image before the rays reach 

 the retina, in which case distinct vision will be pro- 

 cured by interposing a concave lens between the eye 

 and the object, of such a curvature as shall cause 

 the rays that pass through the crystalline lens to meet 

 on the retina. Such is the case with short-sighted 

 persons, but as the crystalline lens becomes more flat- 

 tened as age advances, this defect is in the course of 

 years gradually diminished. It is owing to the same 

 gradual flattening of the crystalline lens that those 

 whose sight has through the early part of life been 

 perfect, are obliged, as old age advances, to employ 



convex lenses, in order to correct the different cur- 

 vature of the crystalline lens, and cause the rays 

 which would meet behind the retina to meet on it. 



The following measure of the crystalline and cor- 

 nea, were taken by doctor Gordon and doctor Brew- 

 ster, from the eye of a female above 50 years of age, 

 a few hours after death. 



Diameter of the crystalline, 378 



Diameter of the cornea, ... 0.400 



Thickness of the crystalline, 0.17-2 



Thickness of the cornea, 0.042 



Measures of the refractive powers of the humours 

 of the same eye : 



Index of Refraction. 



Refractive power of water, 1.335S 



Ditto, of aqueous humour, 1.3366 



Ditto, of vitreous humour ... . . 1.3394 

 Ditto, of outer coat of crystalline, . . . 1.3767 



Ditto, of middle coat of ditto, 1.3786 



Ditto, of central part of ditto, 1.3990 



Ditto, of the whole crystalline, 1.3339 



The eyes of different species of animals differ con- 

 siderably in form and number. All red-blooded 

 animals have two eyes, but two species ; the zemni, 

 or blind rat, and the typlus, or golden mole, are 

 rendered blind by a thin hairy film that covers the 

 organ. In animals that inhabit the lower regions of 

 the atmosphere, including those that move along the 

 surface of the earth, the eye is nearly spherical, 

 the longest axis being from the cornea to the retina. 

 In fishes, the fore part of the eyeball is much flat- 

 tened ; in some the eyeball is like a hemisphere, 

 the anterior part of which is flat, and in others the 

 superior part is likewise flat ; but in birds which 

 occupy the higher regions of the atmosphere, the 

 deviation from the spherical form is in the contrary 

 direction to that of fishes. The aqueous humour is 

 more abundant in high-flying birds than in animals 

 near the surface of the earth, and less still in fish, 

 in which last it is sometimes awanting. The crystal- 

 line lens is most globular in fishes, and flattest in 

 birds. The less spherical the form of the eye is, the 

 stronger i& the sclerotica. In the cuttle-fish the cor- 

 nea is awanting, the iris is of a deep tawny brown 

 colour ; in the mammalia and in birds, it varies from a 

 bright yellow, red, or clear blue, according to the 

 species. In fishes it is of a golden or silvery lustre, 

 and in reptiles it is intermediate in metallic lustre. 

 The pupil is round in man. In the cat kind it is 

 formed by two elliptical segments, the opening run- 

 ning parallel to the direction of the nose. In rumi- 

 nating animals, the horse and the whale, the opening 

 runs transversely, and in the dolphin it is of a heart 

 form. The adaptation of peculiar structure in the 

 eyes of animals to their wants and situations, forms 

 a striking and interesting branch of physiology. 



Eyes of a Portrait. The influence which the as- 

 sociation of contiguous objects has upon our ideas, is 

 strikingly exemplified in the eyes of a portrait. We 

 estimate the direction of the eyes, not only from the 

 position of the ball in regard to the eyelids, but also 

 from the relative position of the remaining features 

 of the face. Dr Wollaston has shown, that the same 

 eyes in a picture, which looks at us, may be made to 

 appear averted from us, if we apply new features to 

 the lower half of the face. The reason why the eyes 

 of a portrait appear to follow us, in all parts of 

 the room, is simply, that the relative position of the 

 features cannot change, so that, if the picture appears 

 to look at us once, it must appear to look at us 

 always. If we move to one side of a portrait, the 

 change which happens is unlike that which would 

 take place in a bust or living face. The picture is 

 merely foreshortened, so that we see a narrower 

 image of a face, but it is still that of a face looking 

 at us. And if the canvass be transparent, the same 

 effect takes place from the back of the picture. 



EYE, in architecture is used to signify any round 



