with special reference to Coronas and Iridescent Clouds. 281 



of the three units, using negative signs when required. 

 Looking at the table below, we see 



(36) =0-48(24) + 1-25(44) -0-02(68) ; 

 which means that the mixture of 0*02 of (68) with unit width 

 of (36) is indistinguishable in hue, depth, or brightness from a 

 mixture of 0'48 of (24 ) with 1-25 of (44). The position of (36) 

 on the diagram is the centre of gravity of weights proportional 

 to 0*48, 1-25, and — 0*02, placed at the three corners (24), 

 (44), and (^8). The brightness of a colour is not indicated 

 by the diagram. 



The most important property of the diagram is the follow- 

 ing. Let us define the brightness of any colour to be the 

 algebraic sum of the corresponding multiples of the three 

 corner units. Then if any colour C be composed of the colour 

 A of brightness a, and of the colour B of brightness /3, its 

 position on the diagram is the centre of gravity of weights a 

 at A, and /3 at B, and its brightness is a + /3. Hence all the 

 colours on any straight line are mixtures of the colours at the 

 two ends of the line, and, in particular, all the points on a 

 straight line drawn from the point white are of the same hue ; 

 the depth or purity increasing as we near the spectrum- 

 colours on the borders of the diagram. 



Now Maxwell has determined the multiples necessary to 

 express unit width at any point of the spectrum in terms of 

 the three corner units. The sum for the whole spectrum 

 must represent white. And, if the relative brightness of dif- 

 ferent parts of the spectrum be altered in a known manner, 

 we can, by introducing the proper factors before summation, 

 find the resultant colour in terms of the three units. Lord 

 Rayleigh used a table, containing twenty-two equidistant 

 points of the spectrum, based on Mrs. Maxwell's observations. 

 From this I have deduced the following abridged table, which 

 is sufficiently accurate for my purpose : — 



Scale-number. 



Wave-length. 



(24) 



(44) 



(68) 



20 



663 



+044 





+004 



28 



608 



+ 117 



+0-32 





36 



563 



+048 



+ 1-25 



-002 



44 



529 





+ 1-00 





52 



500 



-0 06 



+0-51 



+0-28 



60 



475 



-0-05 



+019 



+0-75 



68 



457 







+ 1-00 



76 



441 



+003 





+0-69 



84 



428 







+033 



92 



417 







+0-15 



+201 



+3-27 



+3-22 



