with reference to Cor once and Iridescent Clouds. 171 



tion of diffraction-colours could not be detected with a nicol 

 prism, though readily seen in thin-plate colours. At 20° the 

 variation of brightness, as the nicol is rotated, should in one 

 case be in the ratio 4:1, in the other 4:3 ; at 30°, 7:1 and 7:4 

 respectively. As I have pointed out before, thin-plate colours 

 are most likely to be seen at 20° or 30° from the sun. 



I have recently looked up Fraunhofer's own account of 

 his observations on the light diffracted by a slit*. I find 

 that, though in his formulated statement he says that the 

 deviations of the red bands are in the ratio 1:2:3...., yet 

 what he actually measured were the boundaries between suc- 

 cessive spectra. He describes the first spectrum as ending 

 with red, and the second as commencing with indigo, blue, 

 &c, and remarks that each spectrum fades by insensible 

 degrees into the next. He measured the extreme limits of 

 the first four spectra, and found they corresponded to " re- 

 tardations " of 0-0000211 Paris inch or 0*000057 centim. 

 multiplied respectively by 1, 2, 3, 4. By referring to the 

 continuous curve in my diagram, it will be seen that the first 

 three of these points lie nearly on the line from W to the 

 violet corner, while the fourth would lie nearly in that line 

 produced. Thus he evidently included what I have called 

 purple in the red. 



4. Bishop's Ring f- 



One of the most interesting examples of a diffraction-corona 

 was the great ring round the sun produced by Krakatoa dust. 

 In the report of the Krakatoa Committee of the Royal 

 Society there is a section devoted to this subject, in which 

 two serious blunders occur, and some important consider- 

 ations are omitted. So a few remarks on the subject may not 

 be out of place. 



The corona is generally described as " a whitish silvery 

 patch surrounded by a brownish fringe," or in similar terms j. 

 Prof. Cornu studied the colours carefully and says that in 

 favourable circumstances the order was, "Proceeding from the 

 centre outwardly, clear azure blue, neutral grey, brown- 



* Schumacher's Astronomische Abhandlungen, vol. ii. 1823. 



f Since this section was written I have received from Dr. J. Pernter 

 a copy of his paper "Zur Theorie des Bishop'schen Binges " (Meteoro- 

 logische Zeitschrift, Nov. 1889), in which some of my remarks are 

 anticipated. Pernter deduces the size of the particles from Fraunhofer's 

 measurements of the diffraction pattern of a circular aperture, and obtains 

 results not very different from mine. In this case also Fraunhofer 

 measured the boundaries of the spectra and included my purple in the 

 red. 



% Krakatoa Report, p. 233. 



