48 



UNDULATORY FORCES. LIGH1. 



[OTKVCTUEE OF THE EVE. 



that our readers have become familiar with 

 the laws of reflection and refraction, as well as of the 

 construction and use of different kinds of lenses, we now 

 purpose to apply our knowledge in investigating the 

 structure of the eye. Wo shall also inquire into the 

 different modifications which are found to exist in the 

 eyes of various creatures, and which are essential to tho 

 particular conditions in which they exist. 



We shall look in vain for any greater instance of 

 perfect adaptation of means to an end, than is found In 

 the construction of the organs of sight. As we proceed 

 in our inquiries, wo shall find that every necessity has 

 been provided for; each part is exactly adapted to its 

 purpose ; and that simplicity of construction is, after all, 

 the characteristic of this masterpiece of the handiwork of 

 tho Deity. 



At the head of this section we have introduced two 

 encravings, which give a sectional and general view of 

 the human eye; and we now proceed to describe the 

 uses cf each part . 



We need scarcely state that one engraving represents 

 the eye as seen from the front. By means of Fig. 22, 

 we shall be able to examine each division, and to ascer- 

 tain its separate purpose. 



We commence witli that part which forms the external 

 visual portion of the eye, termed the cornea. This is a 

 transparent horny substance, and encloses what is called 

 the aqueous or watery humour. . It forms the first lens in 

 the eye and refracts rays of light incident on its surface. 

 Referring to Fig. 22, it will be found as marked a 

 therein. On the other side of the cornea, the pupil is 

 next observed, which is an aperture through which light 

 passes into the eye from the cornea. A most adniirable 

 contrivance exists in front of the pupil called the iris, ef. 

 This is a movable diaphragm, which lets in or closes out 

 light from the eye; and which, by its contraction or 

 expansion, regulates the amount of incident rays in their 

 passage to the retina. The student may easily satisfy 

 himself as to the object of the iris, by observing the eye 

 of tho domestic cat during dusk and in full sunshine. 

 When but a dim light exists, the iris will be expanded, 

 and the pupil will appear large. In full daylight, 

 however, the iris is contracted, and the pupil of the eye 

 presents only the appearance of a narrow slit almost, in 

 fact, shutting out light from the inner portion of the eye. 



Beyond the pupil 6, we next observe the crystalline 

 lens c, which has a double convex form. This lens is 

 filled with a fluid of mixed chemical character, and of 

 higher refractive power than that found in the cornea. 

 The centre, g, of the eye forms a cell of a nearly circular 

 form, filled with what has been termed the vitreous 

 humour. 



At the back of the eye, facing the crystalline lens, we 

 find the retina, so called from its net-like construction. 

 This is at II in our illustration. It is on this membrane 

 that the images of objects, whose rays pass through 

 a be, are depicted. It, in fact, forms a kind of screen, 

 very similar in its use to the ordinary screen employed 

 in the exhibition of dissolving views, <tc. 



Surrounding, and next to the retina, is tho choroid 

 membrane k k, which has a black colour, whose office is 

 to absorb the rays of light after they have passed on to 

 the retina. An external coat, the sclerotica, encloses 

 this and will be observed at i. This extends to the 

 cornea, and forms what is called the " white of the eye." 



At the back of the ball of the eye, we observe a pro- 

 jection extending from it. This is called the optic nerve, 

 h and, by its action, the image painted on the retina be- 

 comes perceived by tho mind. In fact, it forms the con- 

 necting link between mind and matter, and is tho channel 

 of nervous sensation between the eye and the brain. 



Tho reader will thus perceive that there are tliree dif- 

 ferent cells, each filled with fluids. These differ in their 

 refractive power ; the index of that of the cornea, a, being 

 1.337 ; that of the crystalline lens, e, 1.384 ; and of the 

 vitreous or centre of the ball, g, 1.339. 



Having thus described each portion of the eye, we 

 proceed to examine the course of a ray of light from an 

 object incident on tho cornea. 



In Fig. 23, retaining the same letters as those we em- 



Fif. M. 



ployed in describing the eye in Fig. 22, we observe the 

 rays of an object, o, proceeding to the cornea, a, and 

 passing to the crystalline lens, c. Here they undergo 

 refraction, just as they would if passing through a double 

 convex lens made of glass ; and eventually we find that 

 an image is painted on the retina, as seen at I I. It may 

 be more satisfactory to the student to try an experiment 

 with the eye of an ox. For this purpose he should re- 

 move the external coatings of the eye, including the 

 retina; and by placing a piece of ground-glass against 

 the cell, which contains the vitreous humour, in the same 

 position that the retina had previously occupied, he will 

 at once perceive an inverted image of any object whose 

 rays have passed through the cornea, the crystalline lens, 

 and the vitreous cell. 



The reader will at once perceive that the image painted 

 on the retina is inverted, and will, doubtless, inquire 

 how it is that wo see all objects in an apparently vertical 

 position? The answer hitherto given to this question is 

 at least unsatisfactory. It is generally stated, that the 

 distinctions between top and bottom, and such expres- 

 sions, are merely tho results of habitual experience ; or, 

 in other terms, we express ourselves in such a manner as 

 accords with the impression of our minds. 'Seeing 

 properly," therefore, becomes a mere question of educa- 

 tion, just as the sense of touch, <tc., may be similarly 

 explained. This, however, is a subject on which it is 

 useless to spend space and time in discussing, and any 

 attempt at analogical illustration would by no means 

 clear away the difficulty of the question. 



One of the most astonishing capabilities of the eye, is 

 its power of adapting itself to view objects at different 

 distances. It is evident that some change must take 

 place in the shape of the lens of the eye, when rays of 

 greatly different positions as to distance pass through it. 

 The mode of action may be twofold. The crystalline 

 lens may be enabled to alter its shape, and therefore its 

 convexity, of its own accord, or the entire ball of tho 

 eye may perform the same operation by the contraction 

 of the muscles surrounding it. Each view has had ita 

 advocates and its opponents. It has been properly re- 

 marked, that the crystalline lens has no muscular struc- 

 ture sufficient for the purpose ; and, on the other hand, 

 that the contraction of tho muscles surrounding the eye- 

 ball would, if exerted sufficiently, tend to disorganise the 

 whole structure of the organ. This subject is one of 

 great difficulty, and not to be easily decided. We are 

 at present much in the same position on the question, as 

 the man who, although his blindness had been removed, 

 could not explain the chain of causes existing between 

 the fiat of his Great Physician and its result. We may 

 explain our position safely and correctly by quoting his 

 words as one of the most logical and satisfactory of 

 conclusions: "One thing I know, that whereas I was 

 blind, now I see ;" and thus we likewise must be content 

 with a simple statement of facts. 



Having confined our attention to tho structure, Ac., 

 common to both eyes, wo have now to speak of another 

 subject, which also presents some difficulty ; namely, how 

 is it that we see singly by means of two eyes f The 

 readiest answer to tlu's is, that if any image be presented 

 to each eye, at such a distance from either that the two 

 images overlap or join each other, then the object will be 

 viewed singly. The student will understand what we 

 mean by trying a very simple experiment. Place the 

 fore-finger between the eyes, parallel to the nose, and not 

 exceeding an inch in distance therefrom the finger will 

 thus be seen double. Now, move the finger gradually 





