SENSE OF VISION. 411 



focus must be considerably lengthened, or the convexity of the eye increased, 

 to cause the rays to meet on the retina: and hence it may be surmised, that 

 the same cause is acting to produce both changes. But that the convergence 

 of the axes is not itself in any way the occasion of the alteration of the focus 

 of the eye, is shown by the fact, that the adaptation is as perfect, in a person 

 who only possesses or uses one eye, as it is when both are employed ; and 

 also by the power, which is possessed by some persons, of altering the focus 

 of the eye by an effort of the will, whilst the convergence remains the same. 

 In regard to the adaptation of the eyes to varying distances, it is further to be 

 remarked, that, when an object is being viewed as near to the eye as it can be 

 distinctly seen, the pupil contracts in a considerable degree.. The final cause 

 of this change, is evidently to exclude the outer rays of the cone or pencil, 

 which, from the large angle of their divergence, would fall so obliquely on the 

 convex surface of the eye, as to be much affected by the spherical aberration ; 

 and to allow the central rays only to enter the eye, so as to preserve the clear- 

 ness of the image. The channel through which it is effected is evidently the 

 same, as that by which the convergence of the eyes is produced, namely, 

 the inferior branch of the third pair of nerves; to the action of which, the 

 sensations upon the retina form the stfmulus, in the same manner as they do 

 to the ordinary variation in the diameter of the pupil under the influence of 

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538. The ordinary forms of defective vision, which are known under the 

 names of myopia and presbyopia, or short-sightedness and long-sightedness, 

 are entirely attributable to defects in the optical adaptation of the eye. In the 

 former, its refractive power is too great; the rays from objects at the usual 

 distance are consequently brought too soon to a focus, so as to cross one 

 another and diverge, before they fall upon the retina; whilst the eye is adapted 

 to bring to their proper focus on the retina, only those rays which were pre- 

 viously diverging at a large angle, from an object in its near proximity. Hence 

 a short-sighted person, whose shortest limit of distinct vision is not above half 

 that of a person of ordinary sight, can see minute objects more clearly; his 

 eyes having, in fact, the same magnifying power, which those of the other 

 would possess, if aided by a convex glass, that would enable him to see the 

 object distinctly at the shorter distance. But as the myopic structure of the 

 eye incapacitates its possessor from seeing objects clearly, at even a moderate 

 distance, it is desirable to apply a correction ; and this is done, by simply inter- 

 posing a concave lens, of which the curvature is properly adapted to compen- 

 sate for the excess of that of the organ itself, between the object and the eye. 

 On the other hand, in the presbyopic eye, the curvature and refractive power 

 are not sufficient to bring to a focus on the retina, rays which were previously 

 divergent in a considerable or even in a moderate degree; and indistinct vision 

 in regard to all near objects is, therefore, a necessary consequence, whilst 

 distant objects are well seen. This defect is remedied by the use of convex 

 lenses, which make up for the deficiency of the curvature. We commonly 

 meet with myopia in young persons, and with presbyopia in old; but this is 

 by no means the invariable rule ; for even aged persons are sometimes short- 

 sighted; and long-sightedness is occasionally met with amongst the young. 

 In choosing spectacles, for the purpose of correcting the errors of the eye, it 

 is of great consequence not to make an over-compensation; for this has a 

 tendency to increase the defect, besides occasioning great fatigue in the employ- 

 ment of the sight. It may be easily found, when a glass of the right power 

 has been selected, by inquiring of the individual, whether it alters the apparent 

 size of the objects, or only renders them distinct. If it alter the size (in- 

 creasing it if it be a convex lens, and diminishing it if it be a concave), its 

 curvature is too great ; whilst if it do not disperse the haze, it is not sufficiently 



