SENSE OF VISION. 751 



man 1 terms it, the mean excursion of a healthy or emmetropic eye from the 

 central point is inwards 45, outwards 42, upwards 34, and downwards 57. 



613. When both eyes are fixed upon an object their axes converge so as 

 to meet it ; and the degree of convergence is, of course, altered by varia- 

 tions in the distance of the object; since, when the object is very remote, the 

 optic axes are virtually parallel, whilst its approach causes them to incline 

 towards each other, and this the more rapidly as the object is brought 

 nearer, the increase being the greatest when it has arrived within the ordi- 

 nary distance of distinct vision. Here, again, we have an example of the 

 automatic nature of voluntary actions ; for the convergence of the eyes that 

 may be produced by this gradual approximation of an object on which the 

 eyes are kept fixed by an exercise of the Will, far exceeds that which most 

 individuals can induce by an effort made directly for the purpose ; and if, 

 when an object has thus been gradually approximated to within a few inches 

 of the nose, the voluntary fixation be intermitted, and the optic axes be 

 allowed to regain their parallelism, they can seldom be brought to converge 

 again upon it, without repeating the Avhole process. It has been thought, 

 from the close accordance between the changes required for the adaptation 

 of the eyes to distinct vision at different distances, and the alterations in the 

 direction of the optic axes which are required to bring the two eyes to bear 

 upon objects at 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 proceed from a common motor impulse. But that the 

 convergence of the axes is not itself in any way the occasion of the altera- 

 tion of the focus of the eye, is shown by these two facts : first, that the adap- 

 tation 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 eyes by an effort of the will, whilst the converg- 

 ence remains the same. 



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

 names of Myopia and Hypermetropia, or "shortsightedness" and "long- 

 sightedness," are entirely attributable to defects in the optical adaptation 

 of the eye. As already stated, the healthy, or emmetropic Eye, when at per- 

 fect rest, is adapted to focus parallel rays on the retina (A, Fig. 263). If, 

 however, a near object be looked at, since the rays proceeding from it are 

 divergent, it -is obvious that without some change in the media the image, 

 instead of being focussed on the retina, would be focussed behind it. The 

 Eye, however, by its power of accommodation, due to the action of the 

 ciliary muscle in causing the lens to become thicker, is capable of effecting 

 this alteration ; and it is by an effort that increases with the proximity of 

 the object that all near objects are seen. In Hypermetropia (c), or long- 

 sightedness, either the Eye as a whole is flattened from before backwards, 

 or the lens is too flat, or both conditions may be present together. In either 

 case, even parallel rays are not brought to a focus on the retina, much less 

 divergent rays, such as those coining from near objects. It requires, in fact, 

 muscular effort to bring parallel rays to a focus on the retina, and a very 

 great effort, which cannot long be maintained, to bring divergent rays. The 

 patient consequently complains of pain after a few minutes' reading or writ- 

 ing. To remedy this state the eyes are placed in a passive state by the in^ 

 stillation of atropia ; the strongest glass is then given, which produces dis- 

 tinct vision of distant objects (objects at 20 feet or more), or, in other words, 

 which will focus parallel rays on the retina. In Myopia (D) either the Eye 

 is too long in the antero-posterior direction, or the lens is too thick, or both 



1 Grafe and Samisi-h, Htindbuch d. ges. Augenheilkunde, Bd. iii, p. 233, 1874. 



