Page:Science vol. 5.djvu/480

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��[Vol. v., No. Ifl

��luniiiig a colored wheel rftpUlly, all bteiul iiiio a grny- illi white. Here you see the 'seven cntors' od Ibe screen ; but, lliough all are liere, I have InCeiitionotly Arranged them so that there ib too much blue, aud the combined result U a very bluish while, which may roughly stand for that of the orlgiual suii-ray. I nov alter the proportion of the color? so as to vir- tually lake out the excess of blue, and the result is colorless or white light. Whlt«, tlien. is not neces- sarily made by combining the ' seven colors,' or any number of them, unless they are there in just pro- portion (which is in effect what Newton himself says); and white, then, may be made out of such a bluish lifcht as we have described, not by putting any thing to it, but by taking away the excess which Ig there already.

Here, again, are two sectors, — oneblue, one orange- yellow wiih the blue in excess, — making a bluish disk where they are revolved. I take out the excess of blue, and now what remains is while. Qere Is the spectrum itself on the screen, but a spectrum which has l>een artiflclally modifleil iti that liie blue end is relatively too strong. I recomblne the colors (by Professor Rood's ingenious device of an elastic mir- ror), and they do not make a pure white, but one tinted with blue. I take out llie original excess of blue, and what remains combines into a pure wiiite. Please bear in mind that when we ' put in ' blue here, we have to do so by straining out other light through some obscuring medium, which makes the spectrum darker, but that, in the ease of the actual sunlight, introducing more blue introduces more light, and makes the spectrum brighter.

The spectrum on the screen ought to be made still brighter in the blue than it is, — tar, far brighter, — and then It might represent to us the original solar spectrum before it has suffered any absorption either In the sun'a atmosphere or our own. The Praun- hofer lines do not appearln it; for these, when found in the solar spectrum, show that certain individual rays have been stopped, or selected for absorption by the intervening atmospheres; and, though even the few yards of atmosphere between the lamp and the screen absorb, it is not enough to show.

Our spectrum, as it appears before absorption, might be compared to an army divided Into numer- ous brigades, each wearing a distinct uniform, — one red, one green, one blue; so that all the colors are represented each by Its own body. If, to represent the light absorbed as it progresses, we supposud that the army advances under a Sre which thins its num- tters, we should have to consider that (to give the case of nature) this destructive Bre was directed chiefly against those divisions which were dressed in blue, or allied colors, so that the army was thinned out unequally, many men in blue being killed off for one in red; and that, by the lime it has advanced a certain distance under fire, the proportion of the men in each brigade has been altered, the red being cam- paratively unhurt. Almost all absorption is thus ■elective in Its action, and often in an astonishing degree; killing off, so to speak, certain rays in prefer- ence to others, as though by an intelligent choice.

���and uot only destroying most of certain divisional (to continue our Illustration), but even picking oal certain dies in each company. Every ray, then, hsa Its own individuality, and on this I cannot too itrongljr Insist; for just as two men retain their personalities under the same red uniform, and one may fall and Uie other survive, though they touch shoulders \a Ibe ranks, so in the spectrum certain parts will be blotted out by absorption, while others next to them may escape.

To illustrate this selective absorption, I put a piece of dldymium glass in the path of the ray. It will, of course, absorb some of the light; but, insteail of dim- ming the whole spectrum, we might almost say it faaa arbitrarily chosen to select one narrow part for action. in this particular case choosing a narrow file near the oraULie, and letting alt the rest go unharmed. In this arbitrary way our atmosphere operates, but in a far

and another there, in hundreds of places all through the spectrum, but. on the whole, much the most in the blue, the Fraunhofer lines being merely part of the evidence o( this wonderful quaai-intelllgent action which bears the name of selective absorption.

Before we leave this spectrum, let us recall one most Important matter. We know that here, befond the red. is solar energy in the form of heat, which we cannot see, but not on that account any less Impor- tant. More than half tlie whole power of the ami ti here invisible, and, if we are to study completely the action of our atmosphere, we shall have to pay great attention to this part, and find out some way of de- termining the loss in it; which will be difBcult, for the ultra-red end is not only invisible, but compreiMed, the red end being shut up like the closed pages of ft book, as you may notice by comparing the n of the red with the width of the blue.

Now, refraction by a prism la not the only way a forming a spectrum. Nature furnishes us color do only from the rainbow, but from non-transparentsttb-l stances, like mother-of-pearl, where the IrrideacentH hues are due to microscopically Sne lines, lately surpassed nature In these wonderful 'g uonsisting of pieces of polished metal, in which W sec at first nothing to account for the splendid [~ of color apparently pouring out from them like !ig from an opal, but which, on examination with a p erful microscope, show lines so narrow tb from fifty to a hundred in the thickness of k I human hair, and all spaced with wonderful preclaloB

This grating Is equal In defiiiing-power to i: such prisms as we have just been looking at, bi light does not show well upon the screen. Youn see, however, that its spectrum differs from t the prism, In that in this case the red eud is expanded as compared with the violet, and the invlalble nltn red is expanded still more; so that this will be tl tiest means for uB to use In exploring that ' dark et nent ' of invisible heat found In the spectrum n' of the sun, but of the electric light, and of all ii descent bodies, and of whose existence we a know from Herschel and Tyndail.

Now. wc cannot reproduce the actual solar spectr

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