Any increase in the number of vibrations of the air produces acute, whilst a decrease attends the grave sounds, and when the waves succeed each other not less than sixteen times in a second, the lowest sound is produced. Light and colours are supposed to be due to a similar cause, and in order to produce the red ray, no less than 477 millions of millions of vibrations must occur in a second of time; the orange, 506; yellow, 535; green, 577; blue, 622; indigo, 658; violet, 699; and white light, which is made up of these colours, numbers 541 millions of millions of undulations in a second. Although light travels with such amazing rapidity, there is of course a certain time occupied in its passage through space--there is no such thing as instantaneity in nature. A certain period of time, however small, must elapse in the performance of any act whatever, and it has been proved by a careful observation of the time at which the eclipses of the satellites of Jupiter are perceived, that light travels at the rate of 192,500 miles per second, and by the aberration of the fixed stars, 191,515, the mean of these two sets of observations would probably afford the correct rate. Such a velocity is, however, somewhat difficult to appreciate, and therefore, to assist our comprehension of their great magnitude, Sir J. Herschel has given some very interesting comparative calculations, and coming from such an authority we can readily believe them to be correct. "A cannon-ball moving uniformly at its greatest velocity would require seventeen years to reach the sun. Light performs the same distance in about seven minutes and a half. "The swiftest bird, at its utmost speed, would require nearly three weeks to make the tour of the earth, supposing it could proceed without stopping to take food or rest. Light performs the same distance in less time than is required for a single stroke of its wing." Dismissing for the present the theory of undulations, it will be necessary to examine the phenomena of light, regarding it as radiant matter, without reference to either of the contending theories. [Page 265]