4.") 4 C. PIAZZI SMYTH ON MICROMETEICAL MEASURES OF 



APPENDIX I. 



PROF. ALEX. S. HERSCHEL'S LETTER ON THE GREEN BAND OF CO, AND ITS 



EXPLICATIONS (eventually Condensed into Plate LXXVIIL). 



Dated November Wth, 1883 ; College of Science, Newcastle-on-Tyne. 



The chart of the green band's lines* is beautiful ; it is quite a page of the spectrum itself 

 much more clearly laid dowu, I am sure, than I have ever seen the tribe of linelets, and I'm 

 astonished how you can have both discovered and plotted so many perfectly ! 



You have far surpassed the sight you gave me last, I find, of the CO band, by dividing 

 " broad " and plenty of the fine lines too, into pairs and triplets. This is a real triumph, that 

 I couldn't well believe possible, when I discovered it by trying to recognise your new map in 

 the drawing and measures that I took of Salleron's CO tube's green band (with five Sulphide 

 of Carbon prisms) in December last; and couldn't make them fit immediately, until I found 

 that you had duplicated and triplicated numbers of the lines that I recorded " broad," " winged," 

 " united pair," &c, only, so that there is a profusion of new dissections of the band that you 

 have managed now to supply for its anatomy ! Aud then Io Tkiumphe ! in searching over 

 the spaces of my "readings" to identify your lines with, I lighted luckily on the key of the 

 construction, which is simplicity itself, and couldn't well be exceeded in the exactness with 

 which your new map reveals it ! Lux in tenebris, what a happy and glorious release you have 

 disclosed to all our uncertainties ! 



I grounded first on this palpable feature of themeasures 



that while the " leaders," and twin-cub followers open out regularly all down the range, it is 

 not so between the twin-cubs and the leader next follovnng them, so that the distances a remain 

 constant, varying from 0159 revns. to 0172 revns. in my readings without any symptoms of 

 expanding, as far as my list went ; so that these " leaders " are simply accompanied on the pre- 

 ceding side by a companion pair that is at an invariable distance from them ! In other words, 

 the " leaders " form a scale-in-chief by themselves, and a little distance preceding it is just such 

 another scale of fainter twins, overlying the former scale. 



How will this relationship, I asked myself, be borne out in the thronged part of the band 

 between its front edge and the " crossing " point, beyond which point as far as the " green giant," 

 it is as plain as the alphabet? — The answer was to take up the constant distance a between any 



* This was merely the raw record-slip taken at the Instrument, in tlio manner which I specially arranged for all the 

 Spectra described in this paper. — C. P. 8. 



