with special reference to Coronce and Iridescent Clouds. 287 



letting the curve, hues included, stand as it is, and pushing 

 the abscissae to the left through a distance X/4. The quarter 

 wave-length varies from 158 at the red corner to 114 at the 

 violet corner ; but it is sufficient to take the mean 136. Even 

 in the first red we find 2R#=600 corresponding to «0 = 485. 



An easy way of seeing these colours to advantage is to 

 lightly sprinkle the object-glasses of a pair of field-glasses 

 with lycopodium seed and direct them to the neighbourhood 

 of the sun. The poorness of the green of the second order 

 compared with that of the third order is well brought out, also 

 the blueness of the first purple compared with the second. 

 The green of the fourth order is quite distinct, and the corre- 

 sponding red just visible. 



The most notable difference between the colours of ice- 

 filaments and those of water-drops is the superiority of the first 

 blue of the latter both in purity and extent. On the whole 

 this agrees with observation, for the best inner blues that I 

 have seen in water-clouds were superior to the best inner 

 blues in ice-clouds. 



Colours of the Sky and Sun. 



To lend additional interest to the diagram I have calculated 

 a few points representing these colours. It is now certain 

 that the blue of the sky and the reddish tinge of the setting 

 sun are mainly due to the scattering of light by particles 

 small compared with a wave-length. The theory of this 

 action is due to Lord Rayleigh *. All that we require for 

 our present purpose is the law that the scattered light varies 

 inversely as the fourth power of the wave-length. When the 

 various parts of the spectrum are compounded in this pro- 

 portion, we obtain the point marked X~ 4 on the diagram. 

 This is a fair approximation to the blue of the sky near the 

 zenith. Lord Rayleigh's preliminary measures gave the sky 

 a somewhat richer hue. 



Since the scattered light varies as \~ 4 it may be shown that 



the transmitted light must vary as e~ liXk * where x ia the 

 length of path and k is a constant, depending on the size and 

 material of the particles and on their number in a given 

 space. The particles will, on the whole, be more numerous 

 where the air is denser, and it is reasonable to take x pro- 

 portional to the mass of air traversed. Capt. Abney has 

 found that if x be expressed in atmospheres and X in 

 thousandths of a millimetre (X© = 0*589), k has the value 



* Phil. Mag. Feb., April, June, 1871, Aug. 1881. 

 Y2 



