448 C. PIAZZI SMYTH ON MICROMETRICAL MEASURES OF 



(3) Outside and far away into the ultra-red, more N bands do extend, as 

 Prof. Herschel saw; but they form a group of their own, beginning in faintness 

 far beyond Prof. Herschel's triplet of lines, or anything that he saw; rising in 

 intensity of light and markedness of physiognomy as they pass that triple of 

 his, and finally subsiding again very materially before they join, but 

 unsymmetrically, the first band of the Thalen group ; for which features 

 please to examine, first the Index Map, and afterwards Plates Nos. LXX. and 

 LXXI. 



(4) Prof. Herschel's triplet of lines is however a very interesting 

 existency, with nothing else like it through all the rest of the X spectrum, 

 and with these two following features in addition, — 



(a) It is shown only, so far as I can make out, in X tubes at very small 

 pressures, say under # 1"; for brighter tubes as to the bands but at greater 

 pressures, say 0*5" to 2*5" show nothing of it. 



(b) This triplet of lines, so anamolous in the tube, and unsymmetrically 

 placed as to the bands of the ultra-red group which pass in front of it, — is 

 nevertheless owing to the X gas ; for it appears to be identical with the triplet 

 of •lines which I discovered last summer in the jar discharge in the open air. 

 The one line seen on that occasion outside the triple, has since then been 

 identified with Oxygen; but the triple having no resemblance to anything in 

 that spectrum, can hardly be of any other than X material, and may be 

 deservedly noted as Herschel's X triple. 



If the large Plates of the X spectrum, Xos. LXXI., LXXIL, and LXXIIL, 

 as observed by myself be now examined, it will be seen that Thalen's Ked 

 series of bands opens a more brilliant portion of this spectrum; and one 

 which, in and after its third band, effloresces into almost an infinity of the 

 closest and most exquisitely defined lines and linelets that were ever packed 

 into a telescopic field of view. Xor were they all revealed even then, for 

 amongst them seemed to be doubles, or other multiples so exceedingly close 

 that they passed the power of my spectroscope, even at the best, to resolve 

 with certainty; and how many degrees further their intricate refinements of 

 structure extend through the residual haze, of which a little still appears, — it 

 is dangerous to speculate. 



Had it not been for the method I elaborated of recording any number of 

 micrometrical places of lines consecutively, and without taking the eye away 

 from the eye-piece, — the attempt to note the exact place of each and every one 

 of such legions of lines in the usual micrometrical manner, would have been 

 hopeless ; for at the average rate of closeness of those which 1 could separate, 

 there are probably 4000 in the first half of the X spectrum alone. 



The brightest example of the X tubes, viz., one at 0*1" pressure, broke 

 down, I regret to say, early in the work, or when I was using it near the D 



