430 C. PIAZZI SMYTH ON MICROMETRICAL MEASURES OF 



widening their distances from each other, and paling and thinning their light, 

 but not losing their definition, they at last came right up to the Green band. 



The Green band as a matter of course began with its " Green-giant " line in 

 magnificent cue ; then came closely packed, but well separated, sharply defined 

 linelets ; then the second leading line, and wider linelets, then the third leading 

 line and after that the long expanding series of sharp linelets, which continued 

 on, and on, and on, until the beginning of the distant Blue band was reached. 



But shortly before that point was arrived at, a broad, faint, grey haze-cloud 

 was passed. I had never seen anything like it, in that spectrum place before. 

 What could it possibly be % 



It turned out to be Glaucous Hydrogen. Not in the shape of the sharp 

 and vividly bright line that it always shows in tubes at smaller pressures, but a 

 mere amorphous bundle of Nebular haze. 



Turning back then to the Red end of the spectrum, there, in the place of 

 the usual Red Hydrogen line, was another broad cloud of faint haze, but of 

 course red in colour. 



This therefore was the reason why even the Orange band of CH, with its 

 ultra thin and sharp linelets, was, for once, not sensibly interfered with by low- 

 temperature H lines. For at that grand pressure of four mercurial inches on 

 the Olefiant gas, — mere nascent Hydrogen could only exist, even with its 

 strongest lines, as a sort of faint vapour, floating like a ghost over certain 

 spectrum places ; and all low temperature H lines being vastly fainter than its 

 two just mentioned high temperature lines (Red, and Glaucous) — their resolu- 

 tion into similarly broad clouds, depressed their intensity of light to beneath 

 the minimum visibile of any eye. 



I spent perhaps half an hour noting these circumstances in the testing 

 spectroscope which has 12° Dispersion A to H, and was planning how I would 

 arrange the great spectroscope of 60° Dispersion, to take advantage of such an 

 unprecedented view of the Orange band of CH, — when I fancied I saw a 

 double line where lately there were only linelets ; then a stronger line appeared 

 between two of that band's leading lines : then another, and another. To my 

 horror they began to look amazingly like low-temperature Hydrogen lines. 

 Turning therefore to the places of the late nebulous clouds of Red H and 

 Glaucous H, I actually saw them slowly gathering themselves together, and 

 settling down as lines into their ancient places. While in half an hour more, 

 Red H and Glaucous H were narrow and vivid exceedingly ; while the whole 

 band of lines and linelets of this poor, persecuted Orange CH was now hidden 

 in a positive jungle of intrusive low-temperature H lines, of a most provoking 

 degree of strength, brilliancy and number. 



My hope then of presenting the Society with a large map of Orange CH, 

 out of that tube, was gone for ever. Because, when decomposition, by spark 



