Cause of Iridescence in Clouds. 427 



In discussing the variation of intensity with distance, we 

 have merely to consider the march of the function I, since 

 the factor (1) is constant. And it appears from the table 

 that, even at 30° from the sun, the intensity has more than 

 half its maximum value. Now it is matter of common obser- 

 vation, that a thin cloud near the sun glows with an intensely 

 bright white light, due probably to irregular diffraction. 

 This white light diminishes rapidly with increasing distance. 

 Hence, on this theory, so far from expecting the brilliance to 

 fall off rapidly from 5° outwards, we might reasonably look 

 for the most obvious colour-effects at 20° or 30°. But we 

 should look in vain. 



The utmost distance at which I have detected the faintest 

 trace of colour is 23°, and this limit is not approached on one 

 day in twenty. 



It is now, I think, clear that Dr. Stoney's is not the true 

 explanation of the whole, or even of the most striking part of 

 the phenomena I have described. Whether it accounts for 

 any part, is a question which can be more satisfactorily dis- 

 cussed after I have set forth my own theory in fuller detail. 



One essential feature in any theory is that the colour of 

 the light sent by the particles should depend on their size 

 only, and not on their orientation. The only form of diffrac- 

 ting particle which satisfies this condition accurately is the 

 sphere. But the long circular cylinder and forms similar 

 thereto, such as the long hexagonal prism, are sufficiently 

 satisfactory for practical purposes. The sphere of course 

 diffracts light in the same way, however it be turned. The 

 prism, on the contrary, to send light at all, must deviate 

 but slightly from the " reflecting plane," but within this 

 plane it can be turned between wide limits without materially 

 affecting the quality of its light. (By the reflecting plane is 

 meant the plane in which a small mirror must lie so as to 

 reflect sunlight to the observer.) 



Let us examine this very important point more closely. If 

 the axis of the prism or filament lie in the reflecting plane, 

 the light from all points of the axis takes equal time from the 

 sun to the eye. So, as far as relative retardation is concerned, 

 we may consider all points on a line parallel to the axis as 

 equivalent. We may with sufficient accuracy replace our 

 filament by a long narrow plate, whose plane passes through 

 the axis and is inclined at the greatest possible angle to the 

 suit's rays. This plate, in turn, may by Babinet's principle 

 be replaced by an equal and similarly situated slit in an 

 infinite opaque screen. As all points in a line parallel to the 

 axis are equivalent, we need only consider the trace on the 



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