286 Mr. J. C. M>Connel on Diffraction-Colours, 



reddish orange. Then a bluish purple extends as far as 5f°. 

 A broad band of blue reaches to 7^°, a poor green to 8^°, 'a 

 fine yellow to 9-J-°, an orange-red to 10^°, a reddish purple to 

 11 £°. The third blue (to 12J°) is greener and decidedly 

 poorer than the second. The third green (to 13^°) is much 

 inclined to yellow. The next noticeable colour is in the pink 

 at 15J°. There is a faint green at 18£°, and a faint pink at 



2H°. 



On Feb. 25th last I noted down some rather fine colours in 

 ice-clouds, in which the tint seemed to depend mainly on the 

 distance from the sun ; in order outwards, yellow, bright red, 

 purple, green, greenish yellow, faint pink. A few minutes 

 later the purple had altered to faint purple, bright blue, and 

 the outer pink was succeeded by purple and green. This is a 

 good illustration of the extent to which the theoretical colours 

 are realized in observation. 



Partly for the sake of comparison and partly on account 

 of its intrinsic interest, 1 give the curve of brilliancy of thin- 

 plate colours, deduced from Lord Rayleigh's figures, with 

 the addition of an ordinate 1 have calculated in the first blue. 

 The light is supposed to fall at a uniform angle on a film of 

 varying thickness. 



When the diffracting particles are spherical the colour- 

 factor is, as we have seen, Jj 2 (z). When z is indefinitely 

 small Ji(V) = z/2 ; so the curve starts from the point X~ 2 . 

 When z is great 



,V(*) = 2„- *-W (*-£); 



so the curve starts somewhat outside the filament curve and 

 after a time comes near coincidence with it, finally oscillating 

 about the same point X. I have calculated the colour* for 

 2Rr// = 600. This is the point marked C x on the diagram, 

 which happens to fall exactly on the filament curve. I think 

 we may conclude that from the first red upwards the colours 

 produced by filaments and by drops will be practically 

 identical. 



In the previous section I have shown that for a not too 

 small distance 6 from the sun and for corresponding colours, 

 L e. w r hen 



ad = 2R0 - X/4, 



the brightness of the water-cloud is about twice that of the 

 ice-cloud. Thus we may make a fair approximation to the 

 brilliancy curve of the former beyond the first purple by 



* Using the table for cf)(n) = 2«- ] Jj(n) given by Airy at the end of 

 ' The Undulatory Theory of Optics.' 



