HELMHOLTZ IN KONIGSBERG 



with Helmholtz, much as many physiologists, especially 

 in Germany, now cultivate physics and mathematics. 



The ophthalmometer, an instrument of too technical 

 a character to be here described, 1 enabled Helmholtz 

 to determine many of the optical constants of the eye, 

 more especially as to the radii of curvature of the re- 

 fractive surfaces, the cornea and lens. He was also 

 able to solve the interesting problem as to the focussing 

 or accommodating mechanism of the eye, by which it 

 adapts itself for distinct vision within a certain range. 



In the normal eye parallel rays coming from infinity 

 are brought to a focus on the retina, and a distinct 

 image is formed. When rays are not brought accu- 

 rately to a focus on the retina, the image is indistinct, 

 by the formation of circles of diffusion around its 

 margin. It is evident that if an object be brought 

 too close to the eye for the refractive media to focus 

 it on the retina, circles of diffusion will be formed, 

 with the result of causing indistinctness of vision, 

 unless the eye has some power of altering its length 

 or the curvature of its refractive surfaces. That 

 the eye has some such power of accommodation is 

 proved by the observation that if we look through the 

 meshes of a net (the net say 3 feet from the eye) at 

 a distant object, we cannot see both the meshes and 

 the object with equal distinctness at the same time. 

 At one moment the meshes will be seen distinctly 



1 Fully described in Appendix D, p. 718, of my Outlines of Physiology. 

 Glasgow, 1878. 



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