SENSE OF SIGHT. 631 



; It is evident, therefore, that the eye cannot perceive distinctly, at the 

 same time, objects which are placed at different distances, but it must 

 fix alternately the nearer and the more remote, and examine each in 

 turn. It is also evident that, in thus bringing alternately the one or 

 the other into distinct view, there is a change of some kind in the con- 

 dition of the eye, by which it adapts itself to the distance or nearness 

 of the object under examination. The observer himself, at the moment 

 of transferring the sight from one object to another, is conscious of a 

 certain effort, by means of which the eye assumes its new condition ; 

 and the alteration thus produced is not quite instantaneous, but re- 

 quires a certain interval for its completion. The process which takes 

 place at this time is the accommodation of the eye for vision at different 

 distances. 



The method by which the accommodation of the eye is effected forms 

 one of the most important parts of the physiology of sight. The facts 

 which have been established by observation in regard to it are as 

 follows : 



I. The change in ocular accommodation for different distances is 

 accompanied by an alteration in distinctness of the images formed upon 

 the retina. 



This is demonstrated by the observations of Helrnholtz with the aid 

 Of the ophthalmoscope. When the retina is brought into view by this 

 instrument, if the person under examination fix his attention upon a 

 distant object, its image is shown upon the retina with distinct outlines; 

 but on changing the point of vision for a near object, the image of the 

 latter becomes distinct, while that of the former loses its sharpness. 

 This indicates that the result in question is not produced simply by the 

 mental effort of the individual, but depends upon a physical change in 

 the refractive condition of the eye. 



II. Accommodation for distant objects is a passive condition of the 

 eye; that for near objects is the result of muscular activity. 



This fact is in some degree made apparent by the nature of the sen- 

 sations accompanying the change. The eye rests without fatigue for 

 an indefinite time upon remote objects; but for the examination of 

 those in close proximity, especially if it be prolonged, a certain effort is 

 necessary, which, after a time, amounts to the sense of fatigue. It is 

 also remarked that solutions of atropine, which, when applied to the 

 eye, cause temporary paralysis of the sphincter muscle of the iris and 

 consequent dilatation of the pupil, suspend, more or less completely, 

 the power of accommodation for near objects, while that for remote 

 objects remains perfect. If both these changes were due to muscular 

 action, it would be necessary to assume that the same substance could 

 paralyze one of the internal muscles of the eyeball, and at the same 

 time leave the other intact, or throw it into a state of permanent rigid- 

 ity ; and there is nothing known which would justify such an assump- 

 tion. Furthermore, there are certain cases of paralysis -of the oculo- 

 motorius nerve, where not only the corresponding external muscles of 



