104 Dr. W. M. Watts on Carbon and Hydrocarbon 



So that, as the pitch of the note sounded rises, the reed 

 diminishes more and more the segment to which it is attached, 

 as compared with the others. Of course this remark is con- 

 fined for the present to cases resembling the hypothetical case. 



The note employed may be either the fundamental or any one 

 of the overtones. As these are in general all inharmonious to 

 each other, only one can be used at a time. But it is probable 

 that, in particular cases, some two or more may become har- 

 monious ; and they would then be capable of combining in a true 

 periodic motion. 



XIII. Carbon and Hydrocarbon in the Modern Spectroscope. By 

 W. Marshall Watts, D.Sc, Physical- Science Master in the 

 Giggleswick Grammar School*. 



IN the January Number of the Philosophical Magazine appears 

 a paper with the above title by Professor Piazzi Smyth, 

 which calls for a few words from me by way of explanation. 



1. Professor Smyth inquires "why, since for cometary work 

 the reference-spectrum should be of feeble intensity, I do not 

 examine it in that shape, viz. as given by the blue base of the 

 flame of a small alcohol lamp, or the all but vanishing globule of 

 flame when a common gas-light is on the point of going out from 

 inanition ?" The answer is simple, that with a spectroscope of 

 six prisms the loss of light is so great that in the spectrum of a 

 blowpipe-flame there would not be more than one line (5165*5) 

 bright enough to be measured, and it was my object to employ 

 as large a dispersive power as possible in order to secure as great 

 accuracy in the determinations as I could. 



The same reason explains why, " although the spectrum con- 

 sists notably and notoriously of five bands, viz. the orange, citron, 

 green, blue, and violet," I only give measurements for three of 

 the bands : the orange and violet bands were not bright enough 

 to be measured accurately. 



2. An equally simple explanation solves the "strange problem " 

 why the lines 5165*5 and 5585*5 are the best-determined. 



5165*5 happens to fall close to the magnesium-line Z>, whose 

 wave-length we know with great accuracy from the labours of 



o 



Angstrom. 



5585*5 happens to be exactly coincident with an iron line in 

 the solar spectrum. The first band of the citron group, although 

 brighter than the second, does not fall near to any marked line 

 in the solar spectrum which could be used as a reference-line ; 

 and its determination is therefore not quite so exact. 

 * Communicated by the Author. 



