8 Light and Sound. [Jan., 



spreads out to form the retina, upon the terminal filaments of which 

 are scattered minute bodies, the so-called " rods and cones." Upon 

 these bodies the luminous waves impinge, and it is considered pro- 

 bable that each accepts only that vibration which synchronizes with 

 its own : in other words, that the perception of light arises from 

 sympathetic vibration.* If this be the case, we should expect that 

 the range of vision would, like hearing, fall within certain limits of 

 pitch. This is well known to be the fact. Albeit the limit of 

 vision is much more restricted than the limit of hearing ; for with 

 the utmost care we are unable to perceive vibrations of the ether 

 beyond the range of an octave ; that is, from the solar line a to l 

 of the spectrum (see Plate). This extent needs, indeed, a practised 

 eye ; ordinarily the range corresponds to the interval termed a 

 sixth in music, that is froni the red to the violet extremity of the 

 spectrum. The lower limit of vision, the extreme red, is produced 

 by ethereal waves recurring 458 millions of millions of times each 

 second; if the undulations be slower than this, they are invisible. 

 The higher limit of vision, the extreme violet, is produced by lumini- 

 ferous waves recurring 727 millions of millions of times each second; 

 if faster, they do not excite the sense of sight. Inconceivable as 

 is this rapidity, these figures are not hypothetical, nor merely 

 probable ; they express absolute facts incontestably established. 



Like the limits of hearing, so these limits of vision vary slightly 

 with different individuals ; some people are capable of seeing farther 

 beyond the red and not so far into the violet, whilst the converse is 

 true with others. Hence, beyond the shadow of a doubt, certain 

 sounds and certain lights perceived by some persons are totally 

 unperceived by others. And when we pass from human beings to 

 the larger animals on the one hand, and to insects on the other, we 

 doubtless have the range both of hearing and of vision considerably 

 extended. We are not aware that the limits of vision in animals 

 have ever been studied ; but analogy and experience lead us to 

 suppose that it differs from our own in many cases. Assuming that 

 the perception of light is due to a sympathetic vibration of the 

 filaments of the retina, it merely needs that these filaments should 

 be capable of vibrating only the one hundred milhonth of a millionth 

 per second slower, and what we call black heat would be perceived 

 as light : and if these filaments could vibrate the same amount 

 faster, what we call the actinic or invisible chemical rays beyond 

 the violet would become directly visible. Now it is highly im- 

 probable that the retina of every animal in creation should be, as it 

 were, tuned to the same pitch as ours ; and if this be so, then forces 

 unrecognized by our senses are perceptible elsewhere. 



The structure of the ear in a calf points to the conclusion 



* This suggestion was first made, we believe, by Melloni, in his ' La Ther- 

 mochrose.' 



