stick. Although many of the properties of the light-waves are also common to all forms of wave-motion, yet others are distinctively due to the waves being of this particular kind. This form of wave, therefore, is to be carefully distinguished from that propagated in a fluid, where there is always a forward and backward motion to the particles. For example, in the familiar case of waves on the surface of water, the particles of water move in circular paths as the waves pass by—that is, each particle moves forward and back exactly as far as it moves up and down. Also in the case of sound-waves, which are waves propagated through a gas, the particles of the air move only forward and back along the line in which the sound-waves are advancing.
The diffraction grating shows that the waves which produce the sensation of light are very minute, and are of every possible length, between the limits of 32,000 to the inch to 64,000 to the inch, measured from crest to crest. This is only one fifth of the total range of wave-lengths that have been measured radiating from the sun, but only those longer than 132000 of an inch, or shorter than 164000 of an inch, ordinarily reach the retina to produce the sensation of light. The diffraction gratmg also shows that the color of light is due directly to the length of the waves, the longest producing the sensation of red light, the shortest of violet, while ranged in between come the various shades of orange, yellow, green, and blue.
Diagram 1 will perhaps give a better idea of the true size and number of the light-waves than is possible from a mere statement of their length and velocity in figures. It represents in section, magnified five hundred diameters, a series of crests of the longest waves that affect the eye as light, passing through a hole in writing-paper, pricked by an ordinary No. 12 sewing-needle, measuring one seventy-fifth of an inch in diameter. It will be noticed that, although the magnified diameter of the hole appears nearly seven inches across, yet the equally magnified crests of the light-waves are still only just far enough apart to be distinctly separated by the eye. On this scale the pupil of the eye would appear nine feet across, and a very good idea of the number of these particular waves, which enter the eye in a continuous stream whenever it receives the light of a distant object, can be had by considering that, if every one of these light-waves passing through the needle-hole in a single second had been represented on the diagram, one behind the other, they would have formed a band extending in the direction of the arrow to a distance of nearly 100,000,000 miles, and to have shown them all on the diagram would have necessitated the paper being long enough to reach from the earth to beyond the sun!
Having once established the fact that the sensation of light is caused by waves originated in the sun and stars, falling upon and irritating the retina of the eye, it of course follows that space must be
