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L. On the Aqueous Lines of the Solar Spectrum. 

 By Josiah P. Cooke, Jun.* 



A CAREFUL examination of the solar spectrum, continued 

 during several months with the spectroscope described in 

 a recent article in this Journal t has led me to the conclusion 

 that a very large number of the more faint dark lines of the solar 

 spectrum, hitherto known simply as tfz'r-lines, are due solely to 

 the aqueous vapour of our air, and hence that the absorption of 

 the luminous solar rays by the atmosphere is chiefly at least 

 owing to the aqueous vapour which it contains. 



The appearance of the Fraunhofer's line D, seen under pre- 

 cisely the same conditions, but with increasing quantities of 

 aqueous vapour in the atmosphere, is shown in figs. 1, 2, 3, and 

 4. The D line is selected because, being a favourite test-object 

 for the spectroscope, its general appearance is well known to all 

 observers. But even more marked changes than those here 

 illustrated have been noticed in others, but chiefly in contiguous 

 portions of the solar spectrum. 



These changes attracted my attention from my earliest obser- 

 vations with the spectroscope ; but with my first instrument and 

 the bisulphide-of-carbon prisms then employed, it was almost 

 impossible to eliminate the effects which might be caused by the 

 variations in the condition of the instrument itself; and as these 

 were known to be very great, it was possible that they might 

 account for all the variations observed. With the improved 

 instrument, however, just referred to, absolute constancy of 

 action is obtained, and all merely instrumental variations avoided. 



A peculiar condition of the atmosphere gave the first clue as to 

 the cause of the changes under consideration. The weather on 

 the 17th of November, 1865, at Cambridge, Massachusetts, w r as 

 very unusual, even for that peculiar season known in New Eng- 

 land as the Indian Summer. At noon the temperature on the 

 east side of my laboratory was 70° F., while the wet-bulb ther- 

 mometer indicated 66°, showing an amount of moisture in the 

 atmosphere equal to 6*57 grains per cubic foot. At the same 

 time the atmosphere was beautifully clear, and the sun shone 

 with its full splendour. I have never seen the aqueous lines of 

 the spectrum more strongly defined than they were on this 

 day, and the total number of lines visible in the yellow por- 

 tion of the spectrum was at least ten times as great as are ordi- 

 narily seen. The appearance of the D line on that day is shown in 

 fig. 4. Between the two familiar broad lines D 1 and D 2 there were 

 eight sharply-defined lines of unequal intensity, which is only 



* From Silliman's Journal for March 1866. 

 t February 1866, p. 110. 



