168 Mr. J. C. M c Connel on Di fraction- Colours, 



ing colours arranged in scattered patches or, at best, broken 

 rings. It is clear, however, that if the observer could only 

 move close up to one of these patches of glowing colour, a 

 complete system of rings would be developed. 



There is a trifling slip in the expression I gave for cos <£', 

 in the discussion of reflexion from thin plates. In that for- 

 mula n should be replaced by n-\-\. It will be observed that 

 this change does not affect any part of the argument. 



The short paragraph on diffraction-colours seen nearly 

 opposite the sun is founded on a miscalculation, and must be 

 withdrawn. These colours appear to be produced in both 

 w^ater- and ice-clouds. Mr. Omond has sent me a list of several 

 cases in which such glories were seen from Ben Nevis, at 

 temperatures far below the freezing-point *. Their theory 

 seems to be in the most unsatisfactory state. I have come 

 across five different explanations, none of which do I consider 

 satisfactory. 



These are the only corrections I have to make, but I find 

 that my description of iridescent clouds has given rise to some 

 misapprehension. Wishing to bring out forcibly the ten- 

 dency of the colours to arrange themselves symmetrically 

 with regnrd to the sun, which is one of the main arguments 

 against their being due to thin plates, I used the w r ords : 

 " Within a circle round the sun, radius about 2°, the clouds 

 are white, or faintly tinged with blue. This circular space is 

 surrounded by a ring of yellow, passing into orange." This 

 led some readers to think that these circles were always to be 

 seen ; whereas, of course, the yellow is not seen unless there 

 is a cloud of suitable density in a suitable position. The 

 yellow ring is often fairly complete, as every one must have 

 noticed round the moon, but it is very seldom a regular circle 

 like those of coronas. 



The greatest distances from the sun at which I have been 

 able to detect colour, since the date of my first paper, are as 

 follows :— St. Moritz, Oct. 20, 1887, 25°; Jan. 9, 1888, 34°; 

 March 10, 29°; Davos, Dec. 27, 37°. But these extreme 

 colours are only faint pinks and greens. In Colorado irides- 

 cences have been observed up to " more than 45° from the 

 sun" ('Nature,' April 21, 1887). 



Since I became interested in these colours I have had few 

 opportunities of observing from the sea-level. Still I have 

 seen enough to feel confident — and this view is supported by 

 the testimony of others — that the phenomenon is nearly as 



* It is at least possible that in such cases the mists consisted of fine 

 drops of water ; and the Ben Nevis observations of fog-bows at all tem- 

 peratures lend support to this suggestion. 



