The waves of the other colours are between these two, while the wave of white light, which is a mixture of them all, is just half-way between the two. Thus was the physical cause of the various hues of colour discovered by this great man, revealing as it does the singular and mysterious analogy between sound and light. The rays of light, like the waves of sound, produce a different effect, according to their length, by causing quicker or slower pulsations in the nerves of sight, just as musical sounds vibrate upon the drum of the ear with different velocities. This is not all, for the relationship between sound and light does not cease here: we have as yet only spoken of the size of the undulations, and have only shown how their dimensions are connected with the sensation of colour; but there are other things to be considered, for on investigation we find that not only do the different coloured waves vary in the length of their undulations, but also in the number that take place in a given time. The perception of sound is produced by the action of the drum of the ear, which vibrates sympathetically with the pulsations of the air that have been originated by the vibrations of the sounding body; and the perception of light is produced in a similar manner by the vibrations originating in a luminous body, and propagating themselves through the luminous ether until they reach the nerves of sight. The number of these pulsations taking place in the eye has been accurately determined in the following manner. Let us suppose that we are looking at a coloured object—let us say, a red railway signal-lamp; from the lamp to our eye there flows a continuous line of luminous undulations; these undulations enter the eye and become depicted on the retina. For every wave that passes through the pupil, there is a separate and corresponding vibration of the optic nerve, and the number of these vibrations that take place in the course of a second can be easily calculated if we know the velocity of light and the breadth of the waves. We have before found that light travels at the rate of 185,000 miles per second; it therefore follows, that a series of undulations 185,000 miles long pass through the pupil every second; consequently the number of vibrations per second is arrived at by calculating how many waves measuring the 40,000th of an inch—that being the length of a wave of red light—are contained in 185,000 miles. The following table, showing the number of waves passing into the eye per second for the different colours, will interest the student:— Extreme red 458,000,000,000,000 waves per second. Red 477,000,000,000,000 " Orange 506,000,000,000,000 " Yellow 535,000,000,000,000 " Green 577,000,000,000,000 " Blue 622,000,000,000,000 "