SICHT. 



SIGHT, DEFECTS OF. 





it with the finger ; but tht exactly identical poinU o( the 



Mtiov miut bo affected by similar points of the two images, it miffi- 



I 



ciently refuted by Mr. Whcatstone's diftcovcry of different perspective 

 projection!) being presented to each eye. Mr. Wheatetone lias also 

 shown that under some circumstances similar pictures falling 

 responding points of the retina; may appear double and in il 

 place*. If to one eye we present in the stereoscope a vertical line B, to 

 the other eye a line inclined some degrees from the perpendicular, we 



A It 



hall see a line the extremities of which appear at different distances 

 before the eyes. If now we draw a faint vertical line, interacting the 

 inclined line at its centre, as at A, and present the two drawings to the 

 eyes as before ; the two strong lines will still coincide, and the resultant 

 perspective line will occupy the same place, while the faint line, 

 though it occupies the same part of the retina as the vertical line B, 

 appears in a different place, namely, at the intersection of the planes of 

 visual direction of the two eyes. In quadrupeds the relation between 

 the identical and non-identical parts of the retime cannot be the same 

 as in man, for the axes of their eyes generally diverge and cannot be 

 made to meet in one point of an object. Muller therefore supposes 

 that there are parts of their retime which are identical, and parts which 

 are not identical, which have no corresponding parts in the other eye, 

 and the relation of the two retina; to each other in the field of vision 

 may be represented as below. Although the theory of corresponding 



point* is the most perfect that has yet been offered in explanation of 

 the phenomena of single and double vision, yet the facts which have 

 been advanced against it by Mr. Wheatstone are sufficient to show that 

 it cannot be adopted without some limitation. Were it even not liable 

 to these objections, it would still only express the cunililiont required 

 for single and double vision, and would leave unexplained the cautc of 

 two impressions giving rise to one sensation. 



There are several curious phenomena connected with the subject we 

 are treating of, some of which we will briefly allude to. If a piece of 

 white paper is viewed through two different coloured glasses, held one to 

 each eye (for instance, through a blue and a yellow glass), the paper is 

 not seen of a green colour, or rather of the whitish colour which would 

 result from the strict mixture of the two colours, but in part blue and 

 in part yellow. Sometimes one colour predominates, sometimes the 

 other ; and if the experiment is long continued, the mingling of the 

 colours, to which there was at first no tendency, becomes more evident. 

 Similar phenomena are observed when two llimili- pictures are 

 viewed in the stereoscope, and it does not appear to be in the power 

 of the will to determine the appearance of either picture or either 

 colour. Thaw facts seem to show that the two eyes are not always 



in action together ; but that at one time the sensations of one eye pre- 

 dominate, and at another those of the other. If the eyes are closed 

 after being fixed for some time on an object, we still continue to seo 

 its image, and the duration of this image, or " spectrum," as it is called, 

 is in a direct ratio with the impression which caused it. 



Spectra left by the images of white or luminous object* are ordi- 

 narily white or luminous, and those left by dark objects dark ; but if, 

 instead of closing the eyes, they are directed upon some white surface, 

 as a sheet of paper, the colours of the spectra are reversed, those which 

 were white when the eyes were closed becoming black when directed to 

 the white surface, and net vend. These phenomena are easily ex- 

 plained. The part of the retina which has received the luminous image 

 remains for some time afterwards in an excited state, while that which 

 has received a dark image is in an uncxcited and therefore 

 more excitable condition. When the eye in this condition is directed 

 towards a white surface, the luminous rays from this surface produce 

 upon the excited parts of the retina a much more feeble impression 

 than upon the uncxcited, and the latter consequently appear inure 

 illuminated. Spectra are sometimes coloured, although the objects 

 which excited them are colourless ; such is the case if the impressions 

 on the retina are very intense, as when produced by the sun's image. 

 But the most curious phenomena relative to ocular spectra arise from 

 the impression of coloured objects on the retina; the spectra con- 

 sequent on these, although coloured, are not of the same noluur as the 

 object, but the opposite or complementary colour. Thus the spectrum 

 of a red object is green, that of a green object red, that of a violet 

 yellow. Sic. There are two modes of explaining these phenomena, t In- 

 leant objectionable of which is the following, offered by Muller : " The 

 perception of any one of the three simple colours consists merely in the 

 retina being in one of those conditions to which it has a tendency when 

 in a state of excitement ; if this condition be artificially excited in an 

 intense degree, the retina acquires an extreme tendency to that of the 

 complementary colour, which consequently is perceived as the ocular 

 spectrum." 



The disappearance of images which fall on the retina at the entrance 

 of the optic nerve, the luminous circles seen on making pressure with 

 the finger on the globe of the eye, and the vascular network which, 

 under certain circumstances, we perceive in our own retina, hnvo 

 already been alluded to and their causes explained in the article 

 EYE, in NAT. HIST. Div. 



SIGHT, DEFECTS OF. Under this head will be comprehended 

 short-sight, long-sight, double vision, and the defective perception of 

 colours, or colour blindness. 



Ifynpla, or Kmr-aijlitidntM (from itva, " 1 shut," and 5i(/, " tin 

 a short-sighted person being in the habit of winking, or half shutting 

 lids when he endeavours to see objects distinctly). 



When the images of surrounding objects are brought to a focus in 

 the eye before they reach the retina, such an eye is myopic ; when, on 

 the contrary, their foci would fall behind the retina, it is presbyopic. 

 Individuals thus affected see all objects indistinctly that are viewed at 

 the ordinary distance of distinct vision ; therefore, to remedy this 

 defect, they bring them within such a distance of the eye as will ensure 

 their images being brought to an exact foci's upon the retina. The 

 point of distinct vision (that is, the distance from the eye at which 

 objects are perceived most clearly) of a perfect eye averages from 15 

 to 20 inches : an eye which cannot discern objects distinctly beyond 

 10 inches may be considered myopic ; but persons affected with a high 

 degree of myopia have their point of distinct vision as near as two or 

 three inches, or even one inch, to the eye. To short-sighted persons 

 all objects appear magnified ; they prefer to read a small type, and see 

 better through a pin-hole in a card than with the naked eye : on the 

 same principle, when they endeavour to see any distant object dis- 

 tinctly, they almost close their eyelids. The explanation of these 

 phenomena is to be sought for in the condition of the eyes themselves; 

 they are generally firmer than usual, their corneie are proternatu rally 

 convex, and their pupils large; hence by diminishing the aperture 

 through which the light is admitted, all but the more direct rays are 

 excluded, and the images on the retina will be more defined. 



The cause of myopia is an over-refractive condition of the eye ; either 

 the cornea or the crystalline lens is too convex, or the humours of the 

 eye generally are too dense or too abundant. 



Treatment. Although it is said that short-sightedness rarely comes 

 on before puberty, our own observations lead us to believe tlmt it is 

 more frequently a congenital defect than is generally imagined. If, 

 however, an incipient case were brought under the notice of the prac- 

 titioner which could be shown to have followed too great an exercise 

 of the eyes upon minute objects, the cure would probably be found in 

 abstaining entirely for a time from such occupations, refraining also 

 from the use of convex glasses, and employing the eyes cliicll \ 

 large and distant objects. But this defect is one that is so little 

 thought of, and is so easily remedied by the use of glasses, that a 

 medical man is seldom called upon to attempt its cure : the only 

 plausible means that have been recommended with this view are, 

 practising the eyes in reading at gradually increasing distances, and a 

 renunciation of such pursuits as require the concentration of vi>in 

 upon near objects. The manner in which concave glasses improve the 

 vision of near-sighted persons', is by causing a divergence of the rays of 

 light before they enter the eye, thus counteracting the over-refractive 



