SENSE OF VISION. 877 



varying degrees of proximity or remoteness, that the former of these movements 

 is in some degree dependent upon the latter, or, at any rate, that the two pro- 

 ceed from a common motor impulse. 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 these two facts ; first, that the adaptation is as perfect in a person 

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

 that some persons possess the power of altering the focus of the eye by an effort 

 of the will, whilst the convergence remains the same. In regard to the adap- 

 tation 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 purpose 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 thus to allow the central rays 

 only to enter the eye, so as to preserve the clearness of the image. The channel 

 through which it is effected is evidently the same as that by which the converg- 

 ence of the eyes is produced namely, the inferior branch of the 3d Pair of 

 nerves ; to the action of which, the sensations received through the retina form 

 the immediate stimulus, in the same manner as they do to the ordinary varia- 

 tion in the diameter of the pupil under the influence of light ; but the volun- 

 tary determination to fix the vision upon the object is the original source of the 

 action. 



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

 names of Myopia and Presbyopia, or "short-sightedness" and " long-sighted- 

 ness," 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 nearest 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 shortest 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 in- 

 terposing between the object and the eye & concave lens, of which the curvature 

 is properly adapted to compensate for the excess of that of the organ itself. 

 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 in- 

 variable rule; for even aged persons are sometimes "short-sighted," and "long- 

 sightedness" is occasionally met with amongst the young. In choosing spec- 

 tacles, for the purpose of correcting the errors of the eye, it is of great conse- 

 quence not to make an over-compensation ; for this has a tendency to increase 

 the defect, besides occasioning great fatigue in the employment 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 (increasing it, if it be a convex 

 lens, and diminishing it, if it be a concave), its curvature is too great ; whilst 



