62 PKOCEEDINGS OF THE AMERICAN ACADEMY 



are visible ; and, as already stated, I never before saw them so sharply 

 defined as on the 17th of November last. 



Several conditions must evidently concur in order that the aqueous 

 lines should be developed in their greatest intensity. In the first place, 

 the air must be charged with vapor not only near the surface of the 

 earth, but also through a great height of the atmosphere. Local causes 

 might greatly increase the amount of moisture in the lower strata of 

 the atmosphere, and aflfect powerfully the hygrometer, which would 

 not, to the same extent at least, influence the indications of the spectro- 

 scope. In the second place, other things being equal, the intensity of 

 the aqueous lines must be strengthened by increasing the length of the 

 path of the sun's rays thi'ough the atmosphere, and this is the longer 

 the lower the altitude of the sun. But then, again, the intensity of the 

 light has such an important influence on the definition of the lines, 

 and the slightest haze in the atmosphere so greatly impairs their dis- 

 tinctness, that I have generally found that the aqueous lines are seen 

 best when the sun is near the meridian. Hence, with an equal amount 

 of moisture in the atmosphere, the late autumn may be a more favor- 

 able season for seeing the aqueous lines than the summer ; for then 

 not only must the solar rays, when most brilliant at noon, traverse a 

 greater extent of air, but, moreover, the atmosphere at this time is 

 usually clearer, and the reflected beam of light which enters the spec- 

 troscope is at times more brilliant than when the sun attains a higher 

 elevation and the light is reflected under less favorable conditions. 



In the examples cited above, the comparisons were made under as 

 nearly as possible the same conditions, so as to eliminate all causes of 

 variation except the one under consideration. Days were selected 

 when the atmosphere was perfectly clear, and the sun's light, so far 

 as I could judge, equally brilliant. Moreover, the position of the spec- 

 troscope and mirror remained unchanged during the whole time. This 

 mirror, which is used for reflecting the sun's light upon the slit of the 

 spectroscope, is so arranged that it can be turned into any position by 

 the observer while his eye is at the eye-piece of the spectroscope, and 

 it was always carefully adjusted at each observation to the position 

 of best definition. The manipulation of the mirror is fully as impor- 

 tant in the use of the spectroscope as it is in microscopy. 



It will be of course understood that the power of developing these 

 faint aqueous lines depends very greatly on the optical capabilities of 

 the spectroscope, and that the figures here given are relative to the 



