344 



membrane ; and that the return to the passive condition, which again 

 adapts the eye to distant objects, is a passive process, following on 

 the previous effort. 



Cramer had therefore only to determine the nature of the active 

 change, by means of which the foci, for a near object, are brought 

 forward to the retina. Now, as Helmholtz had shown, that the ad- 

 aptation of the eye to distance must depend upon a change of some 

 kind in the refractive condition of the humours of the organ ; and as 

 SenfF had previously proved that no change takes place in the curva- 

 ture of the cornea ; and as the ingenious theories of Ludwig and Stell- 

 wag had in no way removed the difficulties involved in explaining 

 how the lens can be moved forward ; there remained only, as a basis 

 for investigation, the hypothesis of a change of form of the lens. 

 This hypothesis, as Volkmann had stated, could only be objected to 

 as insufficient ; but not as involving any contradiction of fact ; and 

 might be verified by more careful and extended observation. '' 



The question, therefore, which Cramer had to determine, was this — 

 is the form of the lens changed in the adaptation of the eye to near 

 objects ? 



Cramer was indebted to Bonders for the fundamental idea on 

 which he proceeded in the solution of this question. Bonders 

 had previously entered on the investigation, but had failed in his ob- 

 servations. He is entitled, however, to the credit of having suggested 

 the employment of the experiment of Purkinje in this inquiry ; and 

 of having subsequently elucidated its successful results. 



Cramer has discovered that in the adjustment of the eye for a near 

 object, there takes place a change in the form of the lens, consisting 

 of an increase in the curvature of its anterior surface, produced by 

 the iris and ciliary muscle, but without alteration in the position of 

 the lens itself ; while the return to its original form for the vision of 

 a distant object is the effect of its own elasticity, which in proportion 

 to the pressure applied, had co-operated in producing the increase of 

 its anterior convexity. He ascertained the occurrence of this altera- 

 tion of form by watching, through an arrangement of his own contri- 

 vance magnifying from 10 to 20 diameters, the change which takes 

 place in the image of the flame of a candle reflected from the anterior 

 surface of the lens during the adjustment of the eye to a near object. 

 The eye having been adjusted to a distant object, and the erect image 

 from the surface of the cornea having been brought nearly to the 



