GASEOUS SPECTRA UNDER HIGH DISPERSION. 443 



taste or the instinct of Hydrogen to shake itself loose from all terrestrial 

 matter, and rise above all the other elements the lightest and most ethereal of 

 them all. 



But my apparatus is still so far from describing all that Hydrogen has to 

 show, and which future observers may discover, — one does not know how 

 soon, — that I make no attempt in the present state of the question to try to 

 develop the kind of order on which its arrangements are founded; — but would 

 beg leave to call attention, by its means, to a general feature touching definition in 

 all bright-lines pectra, viz., excepting some of the fainter lines in the ultra 

 Red, the definition of Hydrogen lines is inimitably fine and sharp through the 

 red, scarlet, orange, yellow, citron, and green until we come near the blue, 

 where a little falling off sensibly occurs. In the further Blue and the Indigo 

 the defalcation increases ; and in the Violet becomes unbearably offensive ; so 

 that what should be a sharp line of light, becomes more like a dull, broadened, 

 or diffuse woollen cord or hazy band. 



Is this change which thus supervenes on approaching the Violet end of the 

 spectrum, a fault of the instrument ; or a quality of Nature ; or a failure of the 

 human eye ? 



Not a fault of the particular instrument built up by me in rather rough and 

 economical fashion, because I have met the same principle of effect in every 

 spectroscope I have looked through,— even including that charming instrument, 

 the Cooke-Monckhoven spectroscope of Professor Tait's Natural Philosophy 

 Laboratory, — a spectroscope whose every adjustment is carried out to perfec- 

 tion, and where nothing seems to have been omitted or neglected. 



Nor is it a necessary, and innate quality of Nature; for exquisitely defined 

 spectrum lines in both the Violet and ultra Violet regions are said to be 

 obtainable, and have I believe often been obtained, though not by me, in the 

 1 medium of Photography. 



Then the failure must arise in the eye. Yes, in the human eye and its 

 total inability to distinguish between Violet as it is in the spectrum, and every 

 other so-called Violet colour under the Sun. For these, so far as I have yet 

 examined, whether in chemical solids, or fluids, flowers or stained glass, are 

 nothing but a mixture of blue and red ; and are allocated to a totally 

 different mean spectrum place than that of violet, by the more than man's dis- 

 criminating power of either the prism, or a diffraction grating. 



In fact, except for the purpose of establishing a sort of border neutral 

 territory, where eye-results may be compared with the blackened imprints on 

 bromo-iodide of silver, so extra sensitive to violet light, — no eye observations 

 should be trusted for minute features and full effects, much beyond Glaucous 

 Hydrogen, — for there Photography can be brought in with advantage, and 

 probably will be, before long for everything, by those eminent scientists who 



