HERMANN VON HELMHOLTZ 



in the latter case, the anterior surface of the lens 

 becomes more convex ; (3) that the posterior surface 

 of the lens also becomes slightly more convex, as 

 for near objects the radius of curvature becomes 

 shorter (5-5 mm.) than for distant objects (6 mm.) ; 

 (4) that when we look at a near object, the distance 

 of the vertex of the cornea from the anterior surface 

 of the lens becomes shorter (3-3 mm.) than for a 

 distant object (3-7 mm.) ; and that during accommo- 

 dation the lens becomes thicker, being 3-8 mm. in 

 thickness for a distant object and 4-3 mm. for a near 

 object, or from, approximately, a th to a ^th of an 

 inch. This amount of change is quite sufficient to 

 bring to a focus on the retina rays of light emana- 

 ting from an object looked at within the limits of 

 accommodation. 



It only remained to explain how the curvature of 

 the lens can thus be changed, and here Helmholtz 

 brought his anatomical knowledge to bear upon the 

 question. Thomas Young, and many of the older 

 observers, thought, erroneously, that the lens was a 

 muscular structure. 1 C. Weber, about 1850, electric- 

 ally excited the eye, and observed the anterior surface 

 of the lens moving towards the cornea. Cramer 

 also electrically irritated the eye, and concluded that 

 a change of form was produced by contractions of 

 muscular structures in the eye itself, and he attributed 



1 Diiyuititionet quae ad facultatem oculum accommodandi sfectant. Mar- 

 burg, 1850, p. 31. 



100 



