444 



SCIENCE 



[Vol. V., No. 121. 



turning a colored wheel rapidly, all blend into a gray- 

 ish white. Here you see the ' seven colors ' on the 

 screen ; but, though all are here, I have intentionally 

 arranged them so that there is too much blue, and 

 the combined result is a very bluish white, which 

 may roughly stand for that of the original sun-ray. 

 I now alter the proportion of the colors so as to vir- 

 tually take out the excess of blue, and the result is 

 colorless or white light. White, then, 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 light as we have described, not by putting 

 any thing to it, but by taking away the excess which 

 is there already. 



Here, again, are two sectors, — one blue, one orange- 

 yellow with the blue in excess, — making a bluish disk 

 where they are revolved. I take out the excess of 

 blue, and now what remains is white. Here is the 

 spectrum itself on the screen, but a spectrum which 

 has been artificially modified so that the blue end is 

 relatively too strong. I recombine 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 the original excess of 

 blue, and what remains combines into a pure white. 

 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 case 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, — far, far brighter, — 

 and then it might represent to us the original solar 

 spectrum before it has suffered any absorption either 

 in the sun's atmosphere or our own. The Fraun- 

 hofer lines do not appear in it; for these, Avhen 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 supposed that 

 the army advances under a fire which thins its num- 

 bers, we should have to consider that (to give the 

 case of nature) this destructive fire 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 time it has advanced a 

 certain distance under fire, the proportion of the men 

 in each brigade has been altered, the red being com- 

 paratively unhurt. Almost all absorption is thus 

 selective 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 not only destroying most of certain divisions 

 (to continue our illustration), but even picking out 

 certain files in each company. Every ray, then, has its 

 own individuality, and on this I cannot too strongly 

 insist; for just as two men retain their personalities 

 under the same red uniform, and one may fall and the 

 other survive, though they touch shoulders in the 

 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 didymium glass in the path of the ray. It will, of 

 course, absorb some of the light; but, instead of dim- 

 ming the whole spectrum, we might almost say it has 

 arbitrarily chosen to select one narrow part for action, 

 in this particular case choosing a narrow file near the 

 orange, and letting all the rest go unharmed. In this 

 arbitrary way our atmosphere operates, but in a far 

 more complex manner, taking out a narrow file here, 

 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 of this wonderful quasi-intelligent 

 action which bears the name of selective absorption. 



Before we leave this spectrum, let us recall one 

 most important matter. We know that here, beyond 

 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 the whole power of the sun is 

 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 difficult, for 

 the ultra-red end is not only invisible, but compressed, 

 the red end being shut up like the closed pages of a 

 book, as you may notice by comparing the narrowness 

 of the red with the width of the blue. 



Now, refraction by a prism is not the only way of 

 forming a spectrum. Nature furnishes us color not 

 only from the rainbow, but from non-transparent sub- 

 stances, like mother-of-pearl, where the irridescent 

 hues are due to microscopically fine lines. Art has 

 lately surpassed nature in these wonderful ' gratings,' 

 consisting of pieces of polished metal, in which we 

 see at first nothing to account for the splendid play 

 of color apparently pouring out from them like light 

 from an opal, but which, on examination with a pow- 

 erful microscope, show lines so narrow that there are 

 from fifty to a hundred in the thickness of a fine 

 human hair, and all spaced with wonderful precision. 



This grating is equal in defining-power to many 

 such prisms as we have just been looking at, but its 

 light does not show well upon the screen. You will 

 see, however, that its spectrum differs from that of 

 the prism, in that in this case the red end is expanded, 

 as compared with the violet, and the invisible ultra- 

 red is expanded still more; so that this will be the 

 best means for us to use in exploring that ' dark conti- 

 nent ' of invisible heat found in the spectrum not only 

 of the sun, but of the electric light, and of all incan- 

 descent bodies, and of whose existence we already 

 know from Herschel and Tyndall. 



Now, we cannot reproduce the actual solar spectrum 



