669 



EYE. 



EYE. 



670 



can conceive to be communicated to the mind by the sum of such 

 indefinite impressions over the whole retina, is a knowledge of the 

 prevailing colour of the object, and possibly a general idea of its 

 direction. But if there were more objects than one, or that one had 

 parts or magnitude, even this inconsiderable addition to the mere 

 sense of light and colour would be impossible. The confusion resulting 

 from the simultaneous impressions of a multitude of pencils of light 

 on the same surface would be partly removed if the seat of perception 

 were placed at the bottom of a cavity capable of being turned to each 

 object or each part of the same object in succession, inasmuch as 

 this would prevent the interference of rays proceeding from parts not 

 actually under contemplation ; but an indistinctness would still remain 

 in proportion to the magnitude of the field of view, only remediable 

 by narrowing the cavity to a mere capillary tube, upon the incon- 

 venience of which we need not enlarge. 



Let us consider what would be the effect of a very simple addition 

 to the cavity. We will suppose it to be closed in front by a dark 

 screen, perforated with a small central hole as in the section repre- 

 sented in fig. 1. 



A 



In this case pencils of rays crossing each other from A and B, the 

 top and bottom of an object, would impinge at a and b upon different 

 parts of the retina. By this means the advantages of a large and 

 a small field of view would be combined, a distributed impression of 

 the object would be produced, and its several parts would be seen 

 separately and in their proper relative situations. The effect may be 

 easily shown by holding a card, pierced with a smooth circular hole, 

 near a taper, and throwing the spectrum upon a wall at a little 

 distance. Such a screen is termed the Iris. 



But still the rays from each point of the object would be diffused 

 over a space instead of being collected upon a separate point of the 

 surface, and the impressions of contiguous pencils would in some 

 degree overlap and confuse each other. This inconvenience might be 

 lessened by contracting the opening, but another cause of indistinct- 

 ness would then be introduced in the diminished admission of light. 



Both evils might be avoided if a lens of a proper construction were 

 fixed behind the screen (as in fig. 2). Pencils diverging from single 

 points of the object would thus be admitted through the opening, 

 which we will call the Pupil, and would be made to converge to single 

 points on the surface, and the impression would now be an exact 

 counterpart of the object, A being distinctly seen in its true place and 

 direction from a, and B from b. 



- A 



But additional provisions would be necessary to bring this arrange- 

 ment to the requisite degree of perfection. In the first place the 

 retina must be adjusted to correspond in shape with the focal distance 

 of the lens. This purpose might be accomplished if the walls of the 

 cavity were composed of flexible materials, by interposing a transparent 

 fluid between the lens and the retina, which, by its uniform distension, 

 would constrain the latter to take and retain the form of a portion of 

 a sphere. 



Again, although the diagram has been otherwise drawn for an 

 obvious reason, our arrangement hitherto supposes the object to be 

 very small, and to be seen directly in front of the eye ; but if oblique 

 as well aa direct pencils are to be brought to a focus, that the lateral 

 vision may be also distinct, a second refracting body, of a proper 

 form, must be placed in front of the lens. This may be done very 

 conveniently, with the further advantage of completing the cavity, by 

 adding a transparent portion to its walls in front of the screen, to be 

 likewise distended with fluid in order to keep it in the shape of a 

 segment of a sphere. (Piy. 3.) 



It is also desirable that the back of the screen and the interior of 

 the cavity should be blackened, that the rays may be extinguished 

 after impact upon the retina, lest any internal reflection should inter- 

 fere with the impressions on other parts. The expediency of this 

 provision in always kept in view in the construction of optical instru- 



ments, and may be made evident by looking at a bright object through 

 a polished metal tube. The colouring-matter is called the Pigmentum 

 Nigrum, or simply the Pigment. [PIGMENT.] 



Fig. 3. 



The only remaining artifice to secure the perfection of the organ 

 that need be mentioned in this synopsis of its most essential provi- 

 sions, is to endow the pupil with the faculty of contraction and 

 enlargement according to the quantity of light. If it were of a con- 

 stant size, more light would be concentrated upon the retina from a 

 bright or a near object than from one comparatively distant or faintly 

 illuminated ; and as the sensibility of the retina must remain the 

 same, the disproportion would occasion dimness of sight iu oiie case 

 and dazzling in the other, and might even impair the nerve. 



We have thus imagined all the parts to be built up in succession 

 that are of primary importance (as far as we know) in the construction 

 of an organ of distinct vision, and the figure to which we have arrived 

 might pass as a tolerably correct diagram of the human eye. 



The laws of light and sensation require that there should be a gene- 

 ral type in the structure of these parts, and a mutual relation among 

 them as to density, form, and position. But this does not preclude 

 much variety ; a difference of position, for instance, may be, and fre- 

 quently is, compensated by a corresponding difference in form or 

 density either of the same or other parts. Hence the problem of dis- 

 tinct vision has many solutions, each perfect in its kind. In fact, 

 nothing can be more diversified in unimportant particulars, or more 

 uniform in those which are essential, than the interior constitution of 

 the eye in different animals : it is never precisely the same in any two 

 species, however closely they may be allied ; but we constantly find 

 the retina, the lens, and the pigment, and generally the iris, inclosed 

 of course in some kind of capsule, transparent in front, and partly 

 occupied by complementary fluids. To this there are some exceptions, 

 which however we believe to be only apparent. Thus the larvae of 

 many insects, some of the microscopic animalcules, and species of 

 Mollusca, have red or black spots upon their surface, which are 

 undoubtedly eyes, and are thought by some to be little more than 

 expansions of an optic nerve beneath a thin coloured membrane to 

 absorb the light, and in some unknown way to distribute its 

 impressions ; whilst others consider them as a congeries of extremely 

 minute but perfectly-formed eyes of the usual construction, of which 

 the pigment alone is visible from its opacity and abundance. 



Globe. The Globe, or Eyeball, contains the parts immediately 

 concerned in vision. It consists of very unequal portions of two 

 spheres of a different size, which have a common circular intersection 

 in a transverse vertical plane, much nearer the front than the back of 

 the eye. The iris, or coloured screen, perforated centrally by the 

 pupil, nearly occupies the situation of this imaginary plane, but is, 

 strictly speaking, behind it. The posterior and larger portion is cir- 

 cumscribed by the sclerotic membrane, except in front, where it may 

 be considered as bounded by the iris : it is rather less than an inch in 

 diameter, and constitutes about five-sixths of the surface of the globe. 

 (Fig. 4, a.) The included space is occupied by the ehoroid membrane 

 and retina, the vitreous and crystalline humours, the ciliary body and 

 processes, and a small part of the aqueous humour. The anterior 

 portion, which forms about a quarter of a sphere, thirteeu-twentieths 

 of an inch in diameter (Jig. 4, b), and occupies the remaining sixth 



'I k 



Section of the spherical surfaces of the human eye, twice the natural size 

 the circles completed in dotted lines. 



a, sclerotic ; b, cornea j c, anterior surface of lens ; d, posterior surface of 

 lens ; e, centre of the eye j /, intersection of the axis of direct vision with the 

 back of the eye ; g, entrance of the optic nerve. 



