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 pulsations 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 conveyed 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 contrivance 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-
