other shall cut the spectrum in two halves horizontally. The upper half will remain unaltered and may be readily compared with the lower half, upon which you will see the spectrum prolonged beyond its ordinary limits. The effect produced is the addition of a splendid band of fluorescent light, which extends over a space of several inches, which but an instant before was a dark mass. I withdraw the prepared paper, and the light disappears; I replace it, and the light shines forth once more; showing us in the most brilliant way that the visible limits of the ordinary spectrum are not the limits of radiant action.
"I plunge a pencil into the solution of sulphate of quinine, and I pass it over the paper. You see that wherever the solution falls, the light bursts forth. The existence of these rays has been known for a long time. Young was familiar with them, and subjected them to experiment; but it is to Professor Stokes that we are indebted for a complete series of researches on this subject. It was he who first made those invisible rays visible, as we have done."
In the same way the Professor proceeded to show that the heat rays were invisible by passing a beam of sunlight through a solution of iodine in spirits of wine, which, although it completely stopped all light, allowed the heat rays to pass uninterruptedly. By collecting these invisible rays into a focus by means of a lens, Dr. Tyndall was enabled to ignite various combustible bodies.
Thus we see the reason why certain rays produce certain effects on the eye, each particular degree of refraction causing a different set of vibrations, resulting in a different sensation for every part of the spectrum, and reproducing the effect of various colours on the optic nerve. In the following chapters we shall conclude our account of the different colours in the spectrum and of the laws of light.
