440 Dr. C. Collingwood on a Horizontal Rainbow 



ally gives the same spectrum as the ordinary Bessemer-flame ; 

 but more commonly a quite different spectrum (fig*. 3) is seen, 

 which reminds one at first of the ordinary carbon-spectrum, but 

 differs from it very remarkably. 



In the carbon-spectrum, which is drawn (fig. 1), each group 

 of lines has its strongest member on the left (i. e. less refran- 

 gible), and fades gradually away towards the right hand : in the 

 spectrum of the spicgel-flame the reverse is the case j each group 

 has its brightest line most refrangible, and fades away into dark- 

 ness on the least-refracted side. A comparison of the drawing 

 of the spectrum of the spiegel-flame (fig. 3) with that of the Bes- 

 semer-flame (fig. 2), will show that they really contain the same 

 lines ; but the general appearance of the spectrum is completely 

 changed by alteration of the relative brightness of the lines. This 

 was shown by direct comparison of the actual spectra. 



There can be no doubt that the principal lines of the Bes- 

 semer-spectrum are due to carbon in some form or other. My 

 own belief is that they are due to incandescent carbon-vapour. 

 The experiments in- which I am at present engaged have already 

 shown the existence of two totally different spectra, each capable 

 of considerable modification (consisting in the addition of new 

 lines) corresponding to alterations in the temperature or mode 

 of producing the spectrum, and each due to incandescent carbon. 

 It is possible that the Bessemer-spectrum may prove to be a third 

 spectrum of carbon, produced under different circumstances from 

 those under which the ordinary carbon-spectrum is obtained ; 

 and the intensity of the dark bands is more probably due to con- 

 trast with the extreme brilliancy of the bright lines than to 

 their actual formation by absorption. 

 University Laboratory, Glasgow, 

 October 25, 1867. 



LVI. Account of a Horizontal Rainbow observed at Sea. 

 By Dr. C. Colltngwood, F.L.S. fyc* 



[With a Plate.] 



ON May 5th of the present year I witnessed a most remark- 

 able phenomenon. I was in lat. 25° 19' S. and long. 

 54° 13' E., the weather very fine and bright, and we were sail- 

 ing with a light breeze. Observing signs of a squall blowing- 

 up from the S.S.E. while I was sitting reading on deck, I got 

 up and went to the port gangway to watch its approach. It 

 was about half-past one p.m., and the sun was therefore in the 

 N.N.W., exactly opposite the approaching squall, upon which 

 was already developed a rainbow of low altitude (12° to 15°). 

 While gazing at it my attention was arrested by a yellowish- 

 * Communicated by the Author. 



