HERMANN VON HELMHOLTZ 



at an object at a distance of 65 metres and ending 

 when the near point is reached at a distance of 

 ^th metre. 



Before these observations were made, the most 

 diverse views had been advanced by physiologists as 

 to the mechanism of accommodation. Some denied 

 that any change took place in the refractive media, 

 and also that any change was necessary ; others, that 

 a change took place in the form of the globe of the 

 eye, the organ becoming compressed by the muscles 

 that move it, so that it became slightly longer for 

 near objects than for those at a great distance ; others, 

 that the contraction of the pupil that takes place 

 when we look at a near object is sufficient to explain 

 the mechanism ; others, that there was a change in 

 the curvature of the anterior surface of the cornea ; 

 others, that the lens was displaced backwards or for- 

 wards by the mucular iris ; while, finally, the true 

 explanation, verified by the facts already stated, had 

 many supporters from the time of Descartes down- 

 wards. The first correct observation was undoubtedly 

 made by Thomas Young. 1 His experiment consisted 

 in looking through wire gauze at a luminous point. 

 An image of diffusion is then seen, traversed by 

 straight lines, which are the shadows of the wires of 

 the gauze. These lines are quite straight when we 

 look at a distant object, but they appear to be curved 



1 Phil. Tram. 1801, vol. i., p. 23, also vol. i. of Young's Works, 

 edited by Peacock. London, 1855. 



