270 Professor S. P. Lamjley [April 17, 



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

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

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

 for brevity, then, that we now unite the more refrangible colours 

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

 terms " orange " or " red." 



All that I have the honour to lay before you, is less an announce- 

 ment 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. 



AVe 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 colours, where, by turning a coloured wheel rapidly, all 

 blend into a greyish white. Here you see the " seven colours " 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 colours so as to virtually take out 

 the excess of blue, and the result is colourless or white light. White, 

 then, is not necessarily made by combining the " seven colours," 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 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 colours (by Prof. Eood's ingenious device of 

 an elastic mirror), and they do not make a pure white, but (me 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 tljan it is — far, far brighter— and then it might represent to us 

 the original solar spectrum before it has suffered any absorj-tion either 

 in the sun's atmosphere or our own. The Frauenhofer 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 absorjition 

 by the intervening atmospheres ; and though even the few yards of 

 atmosphere between the lamj) and the screen absorb, it is not enough 

 to show. 



Our spectrum, as it appears before absorption, might be compared 



