Mr. J. P. Hennessy on some Demonstrations in Geometi'y. 417 



the principles of ' ordinary interference;' and some time after- 

 wards* I applied the same explanation to the coloui'S of a jet of 

 steam observed by Prof. Forbes. 



In one point, however, my view diverges from that of Mr. 

 Reuben Phillips. He names the water-particles which cause the 

 interference " drops of water/' while I believe that they are 

 water-bladders, for which view I have adduced my reasons in a 

 separate papert- 



Besides this, I should like to mention two points with regard 

 to which I have been unable to obtain from the paper of Mr. 

 Phillips a clear notion of the author's opinion. 



(1 .) Among the various colours of the atmosphere there appears 

 to me to exist only two simple originating ones ; namely, the 

 blue colour in all its shades, from dark blue to white, due to 

 interference by reflexion-, and orange-red colour in the corre- 

 sponding shades, due to interference by transmission. The other 

 colours exhibited at times in various portions of the heavens, as, 

 for example, purple or green, I hold to be due to the mixing of 

 the above two colours in their diiferent shades. 



(2.) When clouds appear coloured, I beheve that the colour 

 exhibited is for the most part not formed in the cloud itself, 

 inasmuch as the little bladders generally differ too much in thick- 

 ness to cause the production of a single determinate colour ; but 

 that the light, partly on its way to the cloud, and partly between 

 the cloud and our eye, assumes its colour; even in the appa- 

 rently clear an- there always exist bladders, which, however, are 

 for the most part so attenuated, that they favour in a particular 

 manner the formation of the first colours of interference, namely, 

 blue and orange-red. 



I remain, Gentlemen, 



Very respectfully yours, 

 Berlin, Oct. 13, 1852. R. ClauSIUS. 



LXV. On some Demonstrations in Geometry. 

 By John Pope Hennessy J. 



WERE it possible to give direct demonstration for every 

 pro])osition in Euclid's Elements of Geometry, it would 

 add, if not to the strength, at least to the beauty of that cele- 

 brated chain of reasoning ; for the reductio ad absurdum, as the 



* Die Licht Erscheinuvgen der Almosph'dre, described and explained by 

 R. Clausius. Leipzig, E. B.Schwickcrt, 1850. Also under the title Bei/ro^e 

 zur Meleorologische Optik, jjublished by John Aug. Gruucrt. Part I. No. 4. 

 p. .'396 ; and iu Pogg. Ann. vol. Ixx.xiv. )). 44i>. 



t Pogg. Ann. vol. Ixxvi. p. 101. 



X Communicated by the Author. 

 PliiL May. S. 4. Vol. 4. No. 27. Dec. 1852. 2 E 



