HERMANN LUDWIG FERDINAND HELMHOLTZ. 595 



What is the original form of the energy? What is its final form? and What 

 are the conditions of transformation ? 



To appreciate the full scientific value of Helmholtz's little essay on this 

 subject, we should have to ask those to whom we owe the greatest discoveries 

 in thermodynamics and other branches of modern physics, how many times they 

 have read it over, and how often during their researches they felt the weighty 

 statements of Helmholtz acting on their minds like an irresistible driving-power. 



We come next to his researches on the eye and on vision, as they are 

 given in his book on Physiological Optics. Every modern oculist will admit 

 that the ophthalmoscope, the original form of which was invented by Helmholtz, 

 has substituted observation for conjecture in the diagnosis of diseases of the 

 inner parts of the eye, and has enabled operations on the eye to be made 

 with greater certainty. 



But though the ophthalmoscope is an indispensable aid to the oculist, a 

 knowledge of optical principles is of still greater importance. Whatever optical 

 information he had was formerly obtained from text-books, the only practical 

 object of which seemed to be to explain the construction of telescopes. They 

 were full of very inelegant mathematics, and most of the results were quite 

 inapplicable to the eye. 



The importance to the physiologist and the physician of a thorough know- 

 ledge of physical principles has often been insisted on, but unless the physical 

 principles are presented in a form which can be directly applied to the com- 

 plex structures of the living body, they are of very little use to him ; but 

 Helmholtz, Bonders, and Listing, by the application to the eye of Gauss's 

 theory of the cardinal points of an instrument, have made it possible to acquire 

 a competent knowledge of the optical effects of the eye by a few direct 

 observations. 



But perhaps the most important service conferred on science by this great 

 work consists in the way in which the study of the eye and vision is made 

 to illustrate the conditions of sensation and of voluntary motion. In no depart- 

 ment of research is the combined and concentrated light of all the sciences 

 more necessary than in the investigation of sensation. The purely subjective 

 school of psychologists used to assert that for the analysis of sensation no 

 apparatus was required except what every man carries within himself, for, since 

 a sensation can exist nowhere except in our own consciousness, the only possible 

 method for the study of sensations must be an unbiased contemplation of our 



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