HERMANN LUDWIG FERDINAND HELMHOLTZ. 597 



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 accompanied 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 rolling 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 motions to right and left are not so, being pro- 

 duced 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 during the whole motion the condition that the 

 optic axes shall intersect at some point of the object whose motions we are 

 following. Besides this, the motion of each eye about its optic axis is found 

 to be connected in a remarkable way with the motion of the axis itself. 



The mode in which Helmholtz discusses these phenomena, and illustrates 

 the conditions of our command over the motions of our bodies, is well worth 

 the attention of those who are conscious of no limitation of their power of 

 moving in a given manner any organ which is capable of that kind of motion. 



In his other great work on the Sensation of Tone as a Physiological Basis 

 far the Theory of Music, he illustrates the conditions under which our senses 

 are trained in a yet clearer manner. We quote from Mr Ellis's translation, 

 p. 95 : 



" Now practice and experience play a far greater part in the use of our senses than we are 

 usually inclined to assume, and since, as just remarked, our sensations derived from the senses 

 are primarily of importance only for enabling us to form a correct conception of the world with- 

 out us, our practice in the observation of these sensations usually does not extend in the slightest 

 degree beyond what is necessary for this purpose. We are certainly only far too much disposed 

 to believe that we must be immediately conscious of all that we feel and of all that enters into 

 our sensations. This natural belief, however, is founded only on the fact that we are always 

 immediately conscious, without taking any special trouble, of everything necessary for the practical 

 purpose of forming a correct acquaintance with external nature, because during our whole life we 

 have been daily and hourly using our organs of sense and collecting results of experience for this 

 precise object." 



Want of space compels us to leave out of consideration that paper 011 

 Vortex Motion, in which he establishes principles in pure hydrodynamics which 

 had escaped the penetrative power of all the mathematicians who preceded him, 

 including Lagrange himself; and those papers on electrodynamics where he 



