444 C. PIAZZI SMYTH ON MJCROMETRICAL MEASURES OF 



have of late employed that method in their own special researches, but usually 

 on a far too miniature scale to satisfy present requirements. 



Subject 2. — O on Oxgyen. 



With this gas comes in a change ; a relief perhaps to many persons after 

 the growing complexities of other spectra. 



So far at least as the spectrum has yet been seen and published in 

 vacuum tubes, it is simplicity itself; and though called "the compound-line" 

 Spectrum of Oxygen, that name was given to it merely in deference to a 

 theoretrical idea, in accordance with which the lines ought to have been 

 compound ; or at all events totally dissimilar to what has been termed, by way 

 of pre-eminence, " the line " spectrum of Oxygen ;- — because that is what 

 results from the high temperature of the jar, or condensed discharge of 

 induction sparks, — in contradistinction to the low temperature, or direct 

 discharge, of the simple spark, which we are now dealing with. 



Of this low temperature, " called " compound-line, Spectrum of Oxygen then 

 it is, that the British Association's Report speaks, when it declares it to consist 

 of four lines only ; one in the Red (or Orange rather), two in the Green (or 

 rather one in the Citron and one in the Green), and one in the Blue (or rather 

 between the Indigo and the Violet) ; but the spectrum places of all four have 

 been accurately measured in Wave-lengths, so that they can be easily identified 

 by any one. 



A gaseous emission spectrum then, consisting of four widely apart lines 

 only, must surely be as simple as any one could desire; and the statement is 

 founded on very high authority, viz., a paper by one of the British Association's 

 Committee, printed by the Royal Society, London, in 1879. The author of it too, 

 — being one who not only " fills his own gas-vacuum tubes," but who launched 

 the depreciating accusation against me that I did not perform that operation for 

 myself (pp. 11 and 12), — I shall hardly be allowed, by either of those two great 

 English Associations to put forth any accounts of more lines than their four ; 

 and yet, if my mode of arriving at more than four had been dependent on my 

 filling my own tubes, — would there not have been a chance of my being com- 

 pared to that publie lecturer who, about half a century ago, in London, under- 

 took, in defiance of the doctors, to drink off half an ounce of Prussic Acid, ol 

 deadly strength according to the Pharmacopeia, but stipulated that he must 

 prepare the fluid himself ! 



Before relating however what I have found, and how ; viz., by open methods 

 which should bring out the same result in whatever part of the world they are 

 performed, and have brought out the same in the hands of both French and 

 English workers ; — there is something more to be precisionised in the Royal 



