ACCOMMODATION. 



107 



it is done by accurately measuring the comparative size of 

 images formed by reflection from the anterior surface of the 

 lens. The results obtained by Helmholtz in observations 

 upon three different persons are as follows : ] 



The mechanism of the changes in the thickness and in the 

 curvatures of the lens in accommodation can only be under- 

 stood by keeping clearly in mind the physical properties of 

 the lens itself and its anatomical relations. In situ, in what 

 has been called the indolent state of the eye, the lens is ad- 

 justed to vision at an infinite distance, and is flattened by 

 the tension of its suspensory ligament. After death, indeed, 

 it is easy to produce changes in its form by applying trac- 

 tion to the zone of Zinn. 2 If we remember, now, the ex- 

 act relations of the suspensory ligament, the ciliary muscle, 

 and the lens, and keep in mind the tension within the globe, 

 it is evident that when the ciliary muscle is in repose, the 

 capsule will compress the lens, increasing its diameter and 

 diminishing its convexity. It is in this condition that the 

 eye is adapted to vision at an infinite distance. It is evident, 

 also, that very slight changes in the convexity of the lens 

 will be sufficient for the range of accommodation required. 

 If we fix with the eye any near object, we are conscious of 

 an effort, and the prolonged vision of near objects produces a 

 sense of fatigue. This may be illustrated by the very fa- 

 miliar experiment of looking at a distant object through a 

 gauze. When the object is seen distinctly, the gauze is 

 scarcely perceived ; but by an effort we can bring the eye to 



1 HELMHOLTZ, Optique phymologique, Paris, 1867, p. 157. 

 8 HELMHOLTZ, op, ci(. t p. 151. 



