220 HERMANN VON HELMHOLTZ 



tions, then the position of the eye may be found at any given 

 secondary position if it is moved from the primary to the 

 secondary position by rotation round an axis, which is per- 

 pendicular to the primary and secondary directions of the line 

 of vision. The great importance of this axiom only became 

 apparent through the interpretation given by Helmholtz of 

 Listing's law as the solution for a minimum of the given 

 form. 



In the closing words of this fundamental paper he says, 

 1 1 therefore believe that the law of the movements of the eye, 

 as explained above, is acquired by the use of the eyes, in which 

 we are continually proving the need of the most exact orienta- 

 tion possible, and that the deduction I have made from this 

 need is in the last resort the origin of the law. We should 

 expect that the development of the muscle would eventually 

 enable these movements of the eye, as required by the need 

 of orientation, to be effected with the least possible exertion. 

 The movements of the eye are controlled by the habit arising 

 from the need of orientation, and I do not see the necessity of 

 seeking for anatomical contrivances to account for the law of 

 these movements/ 



The fatigues of his year of office, his lectures, the laboratory, 

 and above all his unbroken scientific work had so affected 

 Helmholtz, that his physician Friedreich urged him at the 

 beginning of the summer holidays to travel, in order to re- 

 cuperate himself as soon as possible. On August 29, 1863, 

 he writes from Heiden (Appenzell) to Bonders: 'As last year's 

 cure at Kissingen was not much good, Friedreich ordered me 

 this summer to drink whey, which I have been doing here at 

 Heiden, and am now going off to the mountains with my 

 colleague Bunsen. I am to meet him on September 3 at 

 Amsteg, and we intend to go round about the Gotthard to 

 Disentis, Airolo, the Tosa Falls, and the Eggischhorn. It is 

 a sad moment when a man is first compelled to become a 

 hypochondriac, and to pay so much attention to his health.' 



The scientific work of the ensuing winter was again devoted 

 entirely to the Physiological Optics, Part III of which was once 

 more to include a number of highly complicated problems ; at 

 the same time he was occupied with numerous public lectures. 



'This winter,' he writes on February 27, 1864, to Ludwig, 



