436 C. PIAZZI SMYTH ON MICROMETRICAL MEASURES OF 



All these have their brightest, hardest edges toward the red end of the 

 spectrum, in so far agreeably with all the CH bands ; while the Citron and 

 Green of the CO bands, fall so nearly on the spectrum places of the similarly 

 coloured Citron and Green CH bands, that beginners may sometimes confound 

 them; or even imagine a physical connection and community between them. 



A very little increase however of Dispersive power with Definition, will 

 show that the CO bands have no leading lines in them, like those which are so 

 prominent in CH bands. The CO bands in fact are made up of nothing but 

 very uniform linelets, and therefore present a smoother, more enamelled 

 looking, surface ; and they are narrower than those of the other compound. 



A far more certain difference however comes out on very highly increasing 

 the spectroscope's powers; for then it will be found that every band of CO has 

 its every linelet of a different construction, or we might almost say material, to 

 any linelet of CH ; and every arrangement of them is different also. This will 

 appear perhaps most strikingly on comparing the Green band of either ; but as 

 we have already given that band of CH, we have only here to picture, describe 

 and discuss the 



Green Band of CO. 



In the Upsala micrometrical view this band is very short, and the linelets 

 of which it is dimly indicated to be composed are coarser and wider apart 

 than those of CH. 



The shortness of the band, as given, merely arises from the circumstance, 

 that at the point where it is cut off towards the violet side, — Green CH 

 (when that is simultaneously present, owing to faulty chemical preparation of 

 CO, or otherwise) comes in, and one band, after that, overlying the other 

 produces confusion. The Upsala philosophers therefore did well, in picturing 

 for green CO by itself, only that little bit of its Green band by whose small 

 breadth it comes out from behind the bright beginning of Green CH ; forming 

 in that way a tiny peninsula of perfectly pure Green CO illumination, which is 

 already somewhat celebrated in spectroscopic story. 



A few years ago this peninsula was thought so very narrow, or minute, a 

 quantity, that it was proposed as a test, much better than the Micrometer 

 measures of that day, to settle whether the carbonaceous spectra of Comets 

 belonged to CH or CO. 



In MM. Angstrom and Thalen's Index Map, the said little bit measures 

 023 of an inch broad, and the shading expended upon it does not claim to be 

 anything more than engraver's ornament. 



In the larger plate of their Mesures Micrometriques the CO peninsula of 

 Green measures 1*4 inches across, and shows 14 indistinct or rounded 

 corrugations, or " fluting.? " of surface. 



