THE SENSES. 



threads of the veil become almost imperceptible, and hardly interfere 

 with the images beyond. 



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

 the same time, objects 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 shifting the sight from one object to the other, 

 there is some change in the condition of the eye, by which it adapts 

 itself to the distance of the object examined ; and the alteration thus 

 produced is not quite instantaneous, but requires a certain time for its 

 completion. This process is the accommodation of the eye for vision 

 at different distances. 



The method by which this is effected is an important part of the 

 physiology of sight. Its principal conditions, so far as they have been 

 ascertained, are the following : 



I. Accommodation for different distances is accompanied by a 

 change in distinctness of the images upon the retina. 



This is demonstrated by the observations of Helmholtz with the 

 ophthalmoscope. When the retina is brought into view by this in- 

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

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

 but on changing his point of vision to a near object, the latter image 

 becomes distinct, while the former loses its sharpness. This indicates 

 that the result is not produced simply by mental effort, but depends on 

 a change in the refractive condition of the ey<-. 



II. Accommodation for distant objects is a passive condition; that 

 for near objects is caused by muscular activity. 



This is in some degree apparent from the accompanying sensation. 

 The eye rests without fatigue for an indefinite time upon remote objects; 

 but examination of those in close proximity, especially if prolonged, 

 requires a certain effort, which, after a time, amounts to fatigue. So- 

 lutions of atropine, which, when applied to the eye, cause relaxation of 

 the sphincter of the iris and dilatation of the pupil, suspend at the 

 same time the power of accommodation for near objects, while that 

 for remote objects remains perfect. Furthermore, in certain cases of 

 paralysis of the oculomotorius nerve, not only the external muscles 

 of the eyeball and the sphincter pupilla? are relaxed, but accommoda- 

 tion is also interfered with ; and in these instances, according to Helm- 

 holtz, the eye remains adapted for long distances. 



III. In accommodation for near objects, the crystalline lens becomes 

 more convex, thus increasing its refractive power. This is the change 

 upon which accommodation is directly dependent. It was first demon- 

 strated by Cramer and Bonders,* by the aid of "catoptric images," or 

 images of reflection in the eye. If a candle flame be so disposed, in a 

 room with dark walls, that its rays fall somewhat obliquely upon the 

 cornea, and at an angle of about 30 degrees with the line of sight, 



* BONDERS, Accommodation and Kefraction of the Eye. London, 1864, p. 10. 



