upon the number of vibrations performed by the molecules of ether. This number, which is very great, differs with each colour, and increases from red to violet; for the extreme red it is 458 millions of millions in a second, and for violet 727 millions of millions. As the velocity of propagation is the same for all the colours of the spectrum, but each corresponds to an unequal number of vibrations, it follows that the length of these vibrations must vary with different colours. It has been calculated that, in the case of red, the length of the vibration is 620 millionths of a millimetre, and for violet 425 millionths.

358. Luminous, heating, and chemical effects of the spectrum.—The various spectral rays differ not only in their colour but also in their luminous power, in the heat by which they are accompanied, and by the chemical effects to which they give rise. It is found that the middle pencils, the yellow and the green, illuminate the most powerfully. Thus the print of a book placed in the yellow pencil is seen more distinctly than in the red or violet.

The heating action of the spectrum is demonstrated by successively placing a very delicate thermometer in the various parts of the spectrum. It is observed that the heat attains its greatest intensity in the red, or rather a little beyond it. The existence of these invisible heat rays, which are less refrangible than all other spectral rays (221), was discovered by Sir J. Herschel, from which fact they are called Herschellian rays.

Passing from the heating action of light to its chemical action, we find that it tends to destroy most vegetable colours, such as wall papers and dyed stuffs, which rapidly fade if exposed to bright light. Some chemical substances are known which are naturally white, and are blackened by the luminous rays, on which property depends the art of photography: there are gaseous mixtures, also, such as that of hydrogen and chlorine, which suddenly explode when exposed to the sun's rays. These chemical effects are not produced equally in all the parts of the spectrum; the greater chemical action is met with in the violet, and even a little beyond.

Figure 314 represents the distribution of the heating, the uminous, and the chemical action of the spectrum; the shaded lines representing the parts of the spectrum which are not visible to the eye, and which, it will be seen, are about equal in length to the luminous parts. The curve I represents the heating effect of the spectrum, from which it will be seen that it is greatest at a little distance outside the visible red; the curve II represents the

-359]

Dark Lines of the Spectrum.

379

intensity of the light, which, it will be seen, is greatest near Fraunhofer's line D in the yellow; the greatest chemical, or, as it is sometimes called, actinic, action is as follows from the form of the curve III, just about the indigo in the visible part of the spectrum. 359. Dark lines of the spectrum.-The colours of the solar spectrum are not perfectly continuous; throughout the whole extent of the spectrum are a great number of very narrow dark lines. They are best observed by admitting a pencil of solar rays into a darkened room through a narrow slit. If at a distance of three or four yards we look at this slit through a flint-glass prism, with its edge held parallel to the edge of the slit, we observe a number of very delicate dark lines parallel to the edge of the prism, and at very unequal intervals.

The existence of these dark lines was first observed by Wollaston in 1802; but Fraunhofer, a celebrated optician of Munich, first

studied and gave a detailed description of them. He'mapped the lines, and denoted the most marked of them by the letters A, a, B, C, D, E, b, F, G, H; they are therefore generally known as Fraunhofer's lines.

The dark line A (see fig. 1 of the coloured plate) is towards the end, and B in the middle of the red; C is in the red but rather nearer the orange ray; D is in the orange ray, E in the yellow, F in the transition from green to blue, G in the indigo, H ir violet. There are certain other noticeable dark lines, s

the red, and b in the green. In the case of the sur'

tions of the dark lines are fixed and definite

ficial lights and of the fixed stars the re1

: