[ 167 ] 



XVI. On Diffraction-Colours, icith special reference to Coronoe 

 and Iridescent Clouds-. By James C. M c Connel, M.A., 

 Fellow of Clare College, Cambridge* 



[Concluded from vol. xxviii. p. 289.] 



3. Corrections and Additions to the First Paper. 



IN the article which I wrote two years ago f on this sub- 

 ject occur some actual blunders and some points which 

 require fuller explanation. The most important blunder has 

 been already mentioned \. In accordance with the usual 

 statement of treatises on Optics, I imagined the central spot 

 of the diffraction diagram to be white, whereas, as a matter 

 of fact, it is coloured with tints similar to those of Newton's 

 first order. Hence what in the calculation I called the first 

 spectrum should really have been called the second ; for 

 second spectrum should be written third, and so on. This 

 does not affect the force of the argument against the theory 

 of thin plates, but it makes a considerable difference in the 

 size of the particles deduced from observation. My first 

 inkling of some error in the previous estimate came from the 

 comparison of the sizes of lycopodium seed deduced from 

 measurements of the diffraction-rings round a candle and 

 from actual observation under the microscope. Applying the 

 true theory of observation, it appears that the diameters of the 

 filaments which produce the brightest colours average 0'008 

 millim. I have measured coronas produced by filaments vary- 

 ing in diameter between 0*01 and 0*045 millim. 



My statement that purple only occurs at the junction of the 

 first two spectra is not strictly correct, for a good purple fre- 

 quently follows the second red. But the two colours are not 

 likely to be mistaken for each other. The first, in general, 

 inclines much to blue ; the second to red. For further 

 description of the colours see the preceding section. 



With regard to the distinction between coronas and irides- 

 cences, it is probable that no hard and fast line can be drawn. 

 The optical explanation of the colour is exactly the same, the 

 only difference lying in the character of the cloud. Still, 

 with ice-clouds at any rate, I have never been in doubt in 

 which class to place the colours. In coronas the rings are 

 complete and perfectly circular, but the tints are in general 

 comparatively poor ; whereas in iridescences we have glow- 



* Communicated by the Author. 



t " On the Cause of Iridescence in Clouds," Phil. Mag-. Nov. 1887. 



\ See vol. xxviii. pp. 274, 280. 



