648 THE POPULAR SCIENCE MONTHLY. 



This experiment only rudely typifies the action of the atmosphere, 

 which is discriminating and selective in an amazing degree, and, as 

 there are really an infinite number of shades of color in the spectrum, 

 it would take forever to describe the action in detail. It is merely 

 for brevity, then, that we now unite the more refrangible colors under 

 the general word " blue," and the others under the corresponding terms 

 " orange " or " red." 



All that I have the honor to lay before you is less an announcement 

 of absolute novelty than an appeal to your already acquired knowledge, 

 and to your reason as superior to the delusions of sense. I have, then, 

 no novel experiment to offer, but to ask you to look at some familiar 

 ones in a new light. 



We are most of us familiar, for instance, with that devised by Sir 

 Isaac Newton to show that white light is compounded of blue, red, 

 and other colors, where, by turning a colored wheel rapidly, all blend 

 into a grayish 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 virtually take out the excess of 

 blue, and the result is colorless or white light. White, then, is not 

 necessarily made by combining the " seven colors," or any number of 

 them, unless they are there in just proportion (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 anything 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 mirror), and they do not make a pure white, but one tinted with 

 blue. I take out the original excess of blue, and what remains com- 

 bines 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 Fraunhofer lines do not 

 appear in 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 



