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NATURE 



{March 8, 1877 



But the scientific importance of the principle of the 

 conservation of energy does not depend merely on its 

 accuracy as a statement of fact, nor even on the remark- 

 able conclusions which may be deduced from it, but on 

 the fertility of the methods founded on this principle. 



Whether our work is to form a science by the colliga- 

 tion of known facts, or to seek for an explanation of ob- 

 scure phenomena by devising a course of experiments, 

 the principle of the conservation of energy is our unfail- 

 ing guide. It gives us a scheme by which we may ar- 

 range the facts of any physical science as instances of 

 the transformation of energy from one form to another. 

 It also indicates that in the study of any new phenomenon 

 our first inquiry must be, How can this phenomenon be 

 explained as a transformation of energy ? What is the 

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

 What are the conditions of the 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 thermody- 

 namics 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 ophthal- 

 moscope, 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 con- 

 struction of telescopes. They were full of very inelegant 

 mathematics, and most of the results were quite inappli- 

 cable to the eye. 



The importance to the physiologist and the physician 

 of a thorough knowledge 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 

 complex 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 con- 

 ditions 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 investiga- 

 tion of sensation. The purely subjective school of psy- 

 chologists 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 un- 

 biased contemplation of our own frame of mind. Others 



might study the conditions under which an impulse is 

 propagated along a nerve, and might suppose that while 

 doing so they were studying sensations, but though such 

 a procedure leaves out of account the very essence of the 

 phenomenon, and treats a fact of consciousness as if it 

 were an electric current, the methods which it has sug- 

 gested have been more fertile in results than the method 

 of self-contemplation has ever been. 



But the best results are obtained when we employ all 

 the resources of physical science so as to vary the nature 

 and intensity of the external stimulus, and then consult 

 consciousness as to the variation of the resulting sensation. 

 It was by this method that Johannes Miiller established 

 the great principle that the difference in the sensations 

 due to different senses does not depend upon the actions 

 which excite them, but upon the various nervous arrange- 

 ments which receive them. Hence the sensation due to 

 a particular nerve may vary in intensity, but not in qua- 

 lity, and therefore the analysis of the infinitely various 

 states of sensation of which we are conscious must con- 

 sist in ascertaining the number and nature of those simple 

 sensations which, by entering into consciousness each in 

 its own degree, constitute the actual state of feeling at any 

 instant. 



If, after this analysis of sensation itself, we should 

 find by anatomy an apparatus of nerves arranged in natu- 

 ral groups corresponding in number to the elements of 

 sensation, this would be a strong confirmation of the cor- 

 rectness of our analysis, and if we could devise the means 

 of stimulating or deadening each particular nerve in our 

 own bodies, we might even make the investigation physio- 

 logically complete. 



The two great works of Helmholtz on " Physiological 

 Optics " and on the " Sensations of Tone," form a splendid 

 example of this method of analysis applied to the two 

 kinds of sensation which furnish the largest proportion of 

 the raw materials for thought. 



In the first of these works the colour-sensation is inves- 

 tigated and shown to depend upon three variables or 

 elementary sensations. Another investigation, in which ex- 

 ceedingly refined methods are employed, is that of the mo- 

 tions of the eyes. Each eye has six muscles by the combined 

 action of which its angular position may be varied in each 

 of its three components, namely, in altitude and azimuth as 

 regards the optic axis, and rotation about that axis. There 

 is no material connection between these muscles or their 

 nerves which would cause the motion of one to be accom- 

 panied by the motion of any other, so that the three 

 motions of one eye are mechanically independent of the 

 three motions of the other eye. Yet it is well known that 

 the motions of the axis of one eye are always accom- 

 panied by corresponding motions of the other. This 

 takes place even when we cover one eye with the fingers. 

 We feel the cornea of the shut eye roUing under our 

 fingers as we roll the open eye up or down, or to left or 

 right ; and indeed we are quite unable to move one eye 

 without a corresponding motion of the other. 

 * Now though the upward and downward motions are 

 effected by corresponding muscles for both eyes, the mo- 

 tions to right and left are not so, being produced by the 

 inner muscle of one eye along with the outer muscle of 

 the other, and yet the combined motion is so regular, 

 that we can move our eyes quite freely while maintaining 



