Page:The wonders of optics (1869).djvu/93

 was always couleur de rose, or in which the inhabitants were continually looking blue! A residence in either of them for a short time would undoubtedly cause us to appreciate the relative value of our own little sun, small as it is in comparison with some of the mighty orbs floating about in space.

The fact that the light of the sun is the source of all the changing hues to be found on the surface of the earth season after season was first discovered by Newton, and his experiments are easily repeated with a very few and inexpensive appliances.

A small round hole is made in the window-shutter of a room, facing the sun, and the pencil of light proceeding from it is allowed to fall upon the surface of a three-sided prism, held in a horizontal position, and placed at a distance of a few inches from the aperture (fig. 5, Frontispiece). The pencil of light does not pass through the prism as if it were a plate of glass with parallel sides, but in virtue of the laws of refraction, of which we have already spoken, it is turned out of its natural course, and is thrown upon the wall in the direction indicated in the figure. The pencil of light is not only turned aside, but it is also widened out into a band which is truly painted with all the colours of the rainbow, every tone and hue being of the most marvellous brilliancy. This long coloured stripe, which constitutes one of the most beautiful sights that the science of optics can afford us, is known to scientific men by the name of the solar spectrum.

Before going into the causes that produce these colours, let us first examine their number and position. Beginning at the top, we shall find that they run in the following order:—Violet, indigo, blue, green, yellow, orange, red. The red being lowest is called the least refrangible of them all; or, in other words, in passing through the prism it was bent less out of its course than