PROFESSOR AT KONIGSBERG 115 



declining to write a textbook on Physiological Physics at 

 the invitation of Vieweg, who in consequence gave the com- 

 mission to Kick, Helmholtz, at Karsten's request, undertook 

 the section on Physiological Optics in his great Encyclopaedia 

 of Physics (a task which unexpectedly required ten years for its 

 completion), and then busied himself in the first place with 

 a new method of determining in the living eye the forms and 

 distances of the refracting surfaces, the cornea and the anterior 

 and posterior surfaces of the lens, in order to define the path 

 of the rays of light in the eye. By April, 1854, he had got 

 so far that after studying Cramer's paper, he could write to 

 Bonders hopefully of speedily determining the curvature of 

 the iris and displacement of the border of the pupil as it occurs 

 in adaptation : 



' I received Dr. Cramer's treatise directly after I had written 

 my first letter to you. I have studied the book, for which 

 again my best thanks, although I found it rather troublesome, 

 as I first had to learn Dutch to read it. Happily your language 

 is so much akin to ours that it is not difficult to understand. 

 Dr. Cramer's work is interesting, and very satisfactory. I did 

 not succeed in experimenting with fresh-killed eyes, because 

 I used rabbits. Cramer's experiments on such eyes show 

 that the iris is necessary to adaptation for near vision, as 

 I had previously surmised. But it still seems to me doubtful 

 whether the iris alone is involved. When the accommodation 

 is for near vision, the edge of the pupil itself bulges forward, 

 while a contraction of the iris alone, i.e. of its radial and 

 circular fibres, which Cramer rightly assumes to occur, would 

 be apt to produce the contrary effect/ 



These difficult optical investigations were now pushed aside 

 by another task, which was forced upon him by an unfortunate 

 incident. At the close of 1853, Clausius published an unjustifi- 

 able attack in the Annalen upon Helmholtz's memoir on the 

 Conservation of Energy, which was a source of great annoyance 

 and distress to Helmholtz, since it emanated from a contemporary 

 and distinguished member of the Physical Society, whom he 

 had known intimately since 1848, and whom he had for a long 

 time been in the habit of meeting almost daily. At the be- 

 ginning of 1854, he refuted the attack in the same journal, 

 under the title ' Reply to the Observations of Dr. Clausius ', 



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