272 Mr. J. C. M c Connel on I)i fraction- Colours, 



For the same substance the right-hand side should be a 

 constant, and it has been shown that it nearly fulfils this 

 condition. 



Mr. Dale now states that for different substances 



Lb — 1 



log-g- =ac, 



where c is a constant independent of the nature of the 

 substance, and a is an integer. This at once leads to the 

 relation v/m = %e ac , which would indicate that the ratios of the 

 volumes to the masses of the molecules are in geometrical 

 progression, or, more shortly, the densities of the molecules 

 are in geometrical progression. 



If, then, Mr. Dale's conclusions are correct this would be 

 the theoretical inference to be drawn from them. 



XXXVI. On Diffraction- Colours, with special reference to 

 Cor once and Iridescent Clouds. By James C. M c Connel, 

 M.A., Fellow of Clare College, Cambridge*. 

 [Plate X.] 



IN a previous paper f I have explained the occurrence of 

 bright colours in certain clouds near the sun on the 

 hypothesis that the light is diffracted by thin needles of ice 

 or by fine drops of water. In the present paper I give a more 

 complete determination of the actual colours produced, based 

 on Maxwell's observations of the colour-relations of the solar 

 spectrum. The first section contains a mathematical investi- 

 gation of the light diffracted by clouds of filaments and of 

 spherules respectively. The second is devoted to calculating 

 the colours and setting them out on Maxwell's diagram. And 

 I have been tempted to mark also on the diagram the colours 

 of the sky and sun. In a future number of this Magazine I 

 hope to publish some additional remarks on iridescent clouds 

 and allied phenomena, including Bishop's ring. 



1. Mathematical Expressions. 



Rectangular Aperture. Point Source. 



We will first take the case of a rectangular aperture 



(sides a b) in an opaque diaphragm inclined to the incident 



light at an angle y, and discuss the illumination on a 



spherical screen of very large radius /, whose centre coin- 



* Communicated by the Author. 

 t Phil. Mag. November 1887. 



