Before concluding, we must add a few more facts relating to the existence of invisible rays at both ends of the spectrum. “The visible portion of the spectrum,” says Dr. Tyndall, in one of his Royal Institution lectures, “simply marks an interval of radiant action, the rays existing in which bear such a relation to our visual organs, as to be capable of exciting in them the sensation of light. Beyond this interval, in both directions, right and left, the radiant action continues to exercise itself, but the rays emitted are dark, in consequence of their exerting no influence on our eye. Those that exist beyond the red ray are capable of producing heat, while those that are beyond the violet excite chemical action. These invisible violet rays can be actually made perceptible to the eye, or, in other words, the undulations or waves proceeding from this end of the spectrum can be made to strike against certain substances and induce luminous vibrations, so as to connect the dark space beyond the violet with a brilliantly illuminated band. I have here a substance capable of effecting this change. The lower half of this sheet of paper has been moistened with a solution of sulphate of quinine, the other half being left in its ordinary condition. I will now hold the paper in such a manner that the line that separates the prepared half from the 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.
CHAPTER V.
THE LAWS OF REFLECTION.—MIRRORS.
When a ray of light falls obliquely on any polished surface, as that of a mirror, a piece of water, a plate of burnished metal, or any other reflecting substance, the ray, like an elastic ball, is immediately projected in a contrary direction to that in which it fell. Moreover, the direction in which it is reflected is at right angles to the surface, and in the same plane as that of the ray in the first instance. This experiment may be tried very easily, and will show the reason for the two following laws.
1. The angle of incidence is equal to the angle of reflection, and vice versâ.
2. Reflection can only take place in one direction—in that of the incident rays, both of which are always in a plane perpendicular to the reflecting surface.
The following figure will assist the student in performing experiments on the reflection of light from flat surfaces.