442 C. PIAZZI SMYTH ON MICROMETMCAL MEASURES OF 



Hardly so, however, for either the number or minute physiognomy of most 

 of its lines ; for whereas the Doctor begins the low-temperature H lines far 

 within, or above, the great Red Hydrogen or C line of the Sun, — I have found 

 them commence far without, or below, Red Hydrogen ; continually increasing 

 too in brightness as they pass that line, and at length join on to Dr Hassel- 

 berg's earliest lines, which are with me very bright. And then again, his scale 

 of 90 inches must necessarily fall lamentably short of a 480 inch spectrum, 

 when it is a question of detecting a close, double, or triple, hitherto recorded 

 as single only. 



This indeed by itself might have been got over, or apologised for; but 

 unhappily the laborious author has allowed his engraver to exert his own taste 

 in doubling and frebling the chief part of his principal lines, and even making 

 a banded group out of the grandly single line Red H, in a manner, and to a 

 degree which must entirely mislead the spectroscopic searcher after micro- 

 metrical truth. 



Now my own original measures of this spectrum, though they cannot 

 pretend to compete with his for accuracy of absolute-spectrum place, are on 

 such a scale that from the earliest Red to the Hydrogen, Violet occupies as I 

 have already mentioned, a continued length of 120 feet. So that if, in this 

 grand hall, you imagine the spectrum strip to begin over the President's Chair, 

 and extend thence continually towards the right, the red and scarlet would 

 reach the end of that wall, the orange would cross that end of the room, the 

 yellow, the citron, the green and the glaucous would occupy all the other long 

 side of the room, the blue would cross its further end, and the indigo, the 

 violet, and the ultra violet would come back and overlap very nearly the 

 beginning of the spectrum's scroll right over the President's head. 



Along with that immense length, truly immense considering it is merely a 

 magnification of a slit about ^J ^ of an inch wide, — you would see nothing of 

 bands of CO with their orderly, closely set regiments of linelets, nothing of the 

 leading lines and fainter linelets of CH, — but only lines and lines and lines 

 again, free, easy and distinct of H. There are some 1625 of them absolutely 

 recorded at the instrument ; generally they are brilliant, well-defined, showy 

 lines ; nowhere very closely packed, but forming all the way along an 

 independent kind of open groups, which have perhaps a certain kind of 

 family resemblance among themselves, but with never any precise repetition 

 between one group and another, either of its strongest single lines, or the 

 occasionally exquisite doubles or trebles which try all the powers of the best 

 spectroscopes yet made to resolve them. In short Hydrogen, in between the 

 positions of its four grand high temperature lines, shows in these almost 

 endless low temperature lines of the tubes, nothing but a saltatory sort of 

 movement, such as an ariel sprite might indulge in, and such as does typify the 



