GASEOUS SPECTRA UNDER HIGH DISPERSION. 425 



the middle of each big bright line, — represented here by a white line on a broad 

 black band, splitting it into two, but foggily and uncertainly. 



This whole result is of course most unsatisfactory and untoward to sharp, 

 micrometric bisection. 



No. 2 exhibits the spark drawn from a solution of Na forming one of the 

 poles, the other being a platinum wire. There is here no continuous spectrum, 

 there is also something sharper and more intense than before in the picture 

 which it gives of the D lines ; but the abnormal central line clown the middle 

 of each standard line is repeated ; and the whole is in a peculiar, crackly, 

 continually exploding, condition, also opposed to very nice bisection. 



No. 3 shows the same identical spark, but altered in quality by the intro- 

 duction of a half gallon Leyden jar. The change is immense, the whole field 

 being now filled with fervid light of the general air glow, also with certain 

 hazy air-bands palpitating in their heated atmosphere, and which I have not 

 attempted to show, while the D lines are still split through the middle and are 

 hazy both inside and out. Finally, 



No. 4 shows the D lines in an end-on vacuum tube. The field here is 

 absolutely black about them ; the D lines absolutely bright, sharp, compact, 

 steady, well defined, and everything that a micrometrical observer could 

 desire. 



In a second tube with the D lines still as above, there were a few faint, 

 low temperature, Hydrogen lines. These were just as sharp and steady as 

 the Na lines, but being vastly fainter were exceedingly thin; so that, over- 

 looking linear, in place of disc-point, figures, the whole field of view gave one 

 the impression of gazing upward into illimitable stellar space, where one star 

 differeth from another in glory, but all of them exist in the quietude of heaven, 

 the calm of eternal peace and distance ineffable. 



A London Objection to my Testimony when using that Method. 



The tube incandescence then is pre-eminently favourable for accurate micro- 

 metrical observations. But before I can expect to have my descriptions of 

 what it has revealed to me listened to elsewhere, it will be necessary to meet 

 openly an accusation lately printed against me by that very same British 

 Association Committee whom I have already on other points alluded to most 

 honourably for their ability, and presumed sense of justice. Yet the following 

 is what appears at p. 12 of their Report for 1880, published in London, when 

 speaking of certain of my observations at that date in vacuum tubes : — 



" Professor Piazzi Smyth has however not filled his own tubes, and we must 

 be careful not to attach too much value to the labels put on vacuum tubes by 

 the glass-blower who has filled them." 



