Wilson. — Analogy between Light and Sound. 511 



First, as to sound. Sixteen pulsations in a second are 

 necessary to produce the lowest musical sound. Below that 

 number, the pulsations, being distinctly heard, form no musical 

 sound, only noise. The highest musical sound which our ears 

 are capable of receiving requires about forty thousand pulsa- 

 tions in a second. Above that number a shrill noise or whistle 

 is the result. Now, if we take the seven notes of any diatonic 

 scale we shall find that the number of pulsations producing the 

 seventh note, counting upwards, is all but double the number 

 required to produce the lowest or first note. Thus, middle C 

 on the piano requires 258 - 7 pulsations in a second; and the 

 seventh note of the ascending scale of C — viz., B — takes 488-2 

 pulsations. The octave of any note takes exactly double the 

 number of the first ; therefore the octave of middle C takes 

 517-4. This is the pitch adopted by the Paris Conservatoire, 

 and quoted by Deschanel in his work on " Sound and Light." 

 There are three prominent notes in the scale, which form what 

 is called the " common chord "; these notes are the first or 

 keynote, the third or mediant, and the fifth or dominant. 

 These sounds can be produced by striking any key on the 

 piano very forcibly, with the loud pedal down, when the third 

 and fifth of that note will be heard sounding faintly after it. 

 In the scale of C, for instance, C, E, G, form the common 

 chord. This satisfies the ear by itself, and is called a concord, 

 whereas combinations of other notes in the scale are not so 

 satisfactory to the ear, and others, again, produce complete 

 discord. 



Let us now turn to the consideration of how light is con- 

 veyed to the eye through the medium of ether. 



The vibrations of light are exceedingly rapid. The lowest 

 number that can make any impression on the retina of the 

 eye as light is computed to be about four hundred and fifty 

 billions in a second. This produces the sensation of red, 

 which is the lowest colour in the spectrum. The highest 

 colour, ultra-violet, requires about eight hundred billions, 

 which, you will observe, is nearly double the number required 

 for the formation of red. Above eight hundred billions in a 

 second only chemical action is the result ; below four hundred 

 and fifty billions, heat. 



I will here give a passage which Shellan quotes from 

 Dove, in his " Spectrum Analysis," page 65. He says, — 



" Dove describes in his own ingenious manner the course 

 of the vibrations as they produce successively sound, heat, and 

 light, as follows : ' In the middle of a large darkened room let 

 us suppose a rod set in vibration, and connected with a contri- 

 vance for continually augmenting the speed of the vibrations. 

 I enter the room at the moment when the rod is vibrating four 

 times in a second. Neither eye nor ear tells me of the pre- 



