with special reference to Coronce and Iridescent Clouds. 283 



At the central point of the diffraction-pattern (af//=0) the 

 brightness is a maximum (instead of being zero as in the case 

 of a thin plate, when V = 0), and the colour-factor is X~ 2 . 

 About 250 the curve passes very near white, on the side 

 towards blue-green. The colour then becomes yellowish, 

 gradually improving, till at 450 a very fine orange is at- 

 tained. And so on through the well-known series. When 

 the retardation is large the curve approaches nearer and 

 nearer to white, and, in the case of diffraction, the brightness 

 diminishes indefinitely. 



Along the diagonals of the pattern, where the colours are 

 the purest, the factor is 



I have determined two points on the curve, Dj in the first red 

 (af// = 500), and D 2 in the second green (af// = 810). The 

 curve starts from the point X -2 , and afterwards for several 

 sweeps keeps outside of the thin-plate curve. The first orange- 

 yellow and the second blue of the latter admit of but little 

 improvement. But in the first red, which borders the central 

 spot, the diagonals are far superior to the principal directions. 

 We now come to the main object of the present paper, the 

 colours of iridescent clouds, formed by needles of ice. The 

 colour-factor is 



. . niraP 

 X sin 2 -~. 



Comparing this with the factor for thin plates we see that the 

 greater wave-lengths have an advantage. So the curve is, on 

 the whole, displaced from the violet and towards the red. 

 This curve is laid down on the diagram with a continuous 

 line. Points actually determined are marked with small 

 crosses, with the values of a%/f annexed in bold figures*. 

 The rest of the curve has been drawn by comparison with the 

 thin-plate curve. The curve starts at the point X" 1 , so the 

 central blue though bright is very impure. On the whole 

 the colours are superior to those of thin plates. The reds are 

 distinctly better, especially the third red. The second blue 

 is nearly as good, though the third is decidedly inferior. The 

 third and fourth greens are about on a par for purity, but 

 more inclined to yellow, while the first and second yellows 



* The calculations were inadvertently made for X 2 sin 2 (nag/fh), and 

 the points given are put halfway between those thus found and the 

 corresponding- points on Lord Kayleigh's curve. All the calculations 

 were made with a slide rule reading to T fo. 



