VISUAL, GRATING AND GLASS-LENS, SOLAR SPECTRUM. 537 



than in the first two ; with the effect of spoiling the purity and beauty of 

 spectrum colour, but of blackening the lines, without I hope disturbing their 

 position for differential measures such as mine. 



Part VI. — Appearances and Disappearances of Terrestrial Water- 

 Gas lines in the Solar Spectrum. 



Bat the most powerful cause of all, for altering the very physiognomy of 

 some districts of the spectrum, is due to the invisible vapour, or gas, of water 

 dissolved in the lower strata of the atmosphere ; and thickening or thinning 

 certain lines, or making new ones, according to its varying amount from day to 

 day, from season to season, and from one country to another. In the dry 

 climate of Portugal in June, and near Noon-day, I could only see a few 

 mediocre lines occupying the spectrum place of " little a " and its preliminary 

 band — both of them due to water-gas. So likewise was it in Edinburgh during 

 the early days of May, in the present year 1885, when the air was both cold 

 and dry ; or with a temperature of 46°, and grains of water-gas in a cubic foot 

 of air = 2 - 6 only. But in Madeira, that "Ocean-flower" fed by perpetual 

 exhalations from the warm currents around it, with a July temperature oi 

 72° '9, and grains of water-gas = 6 "49, — both those constellations of spectral 

 water-vapour lines, even in a high Sun, were rich exceedingly in thick, black 

 groups inimitably defined on brilliant red light, so as to form quite an inspirit- 

 ing sight to have beheld once in one's life. 



And at Winchester, in both June and July, I am bound to confess that the 

 said constellations put on a most respectable appearance, under the usual 

 impregnation of the air in that locality and at that season to the extent of 4*5 

 and even 5'0 grains of water-gas to the cubic foot; chiefly thickening lines 

 already known both to exist, and to represent water vapour. 



The puzzling manner however in which the thinnest class of Fraunhofer 

 lines, if of water-gas, may either appear, or entirely disappear in the spectrum, 

 in places where they might have been entirely unexpected by the observer, — 

 was first and well described by Professor Josiah P. Cooke, of Cambridge, 

 Mass., U.S., in a contribution to the American Journal of Science in November 

 1865. For, confining himself there, to the narrow space between the two Solar 

 and therefore permanent and steady D lines in the Yellow, — he showed how 

 the number of thin interstitial lines increased, just as the weight of water-gas 

 in the cubic foot of air gradually enlarged from 081 to 6 57 grains. 



Professor Cooke further reasoned well on the Annual Maximum of such 

 lines occurring in the American autumn, when the weight of such transparent 

 water-gas dissolved in the air, but not forming clouds, or interfering with the 

 brightness of the Sun, conies also to its maximum. 



Now this weight of water-gas in the air, is quite a different matter to the 



