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 XXI. Intelligence and Miscellaneous Articles. 

 NOTE ON THE SPECTRUM OF BRORSEN's COMET. 

 BY PROF. C. A. YOUNG, OF PRINCETON, N. J. 

 A ITER several unsuccessful attempts, I have at last, on April 1st 

 J -^- and 2nd, obtained fairly satisfactory observations of the spec- 

 trum of this comet. It consists of three bands, like the spectra of all 

 the other comets hitherto observed, the bands beiug well defined at 

 the lower (least refrangible) edge and fading out towards the upper. 

 The spectrum is so faint that observation is very difficult, and I 

 was able to determine the position of only one of the bands — that 

 in the green, which is much brighter than the other two. 



The instrument employed was the 9^ -inch refractor of our new 

 observatory, armed with a one-prism spectroscope of sufficient dis- 

 persive power to separate the D lines clearly : the eyepiece has a 

 micrometer which carries a bar thick enough to be seen on the 

 background of even a very feeble spectrum. The observation was 

 made by placing the bar so that the bright edge of the band should 

 be just visible as a thin line, the rest of the band being occulted. 

 The instrument has also a scale like that of the ordinary chemical 

 spectroscope ; and the position of the micrometer-bar is determined 

 both by the reading of the micrometer-screw and by the reading of 

 the scale, illuminated for a moment after the bar has been set. 



On April 1st I got three scale-readings — respectively 99-9, 100-0, 

 and 100*4, the value of one scale-division in this part of the spec- 



o 



truin being very nearly 25 units of Angstrom's scale, or about 

 double the distance between the extreme lines of the b group, the 

 readings decreasing with the wave-length. 



Just before dark, b n in the spectrum of daylight coincided with 

 100 on the scale; also, immediately after the third pointing and 

 without disturbing the telescope, spectroscope, or micrometer, the 

 flame of a Bunsen burner was presented to the slit ; and the lower 

 edge of the green band in the well-known spectrum of this flame 

 was found to show itself at the edge of the occulting bar precisely 

 where the comet- spectrum had been. We may therefore fairly 

 conclude that the lower edge of the central band in the comet-spec- 

 trum had a wave-length of very nearly 517 millionths of a milli- 

 metre. The observation of April 3rd confirms this, though but a 

 single reading could be obtained. The only special interest in this 

 observation lies in the fact that in 18 G8 Mr. Huggins obtained a 

 somewhat different result for this same comet. 



In an elaborate paper published some years ago by Vogel in 

 Poggendorff 's Annalen, upon the spectra of comets, he comes to 

 the conclusion that there are several different kinds of cometary 

 spectra, the differences lying merely in the wave-length of the 

 bands. But he seems to have reached this conclusion by assigning 

 rather too high a degree of accuracy to the observations. With 

 the exception of Brorsen's comet, it would seem that the discre- 

 pancies between the different results are entirely within the range 

 of probable error, and that there is no valid reason for supposing 



