260 C. S. Cook — Spectrum of Aqueous Vapor. 



The variation in the comparison lines is effected by turning 

 the micrometer screw or collar "<?." The rotation of this 

 collar causes a diaphragm bearing the silk fiber to slide within 

 the observing tube and so remove the cross line from the focua 

 as far as desired. As the fiber is withdrawn from the focus, 

 the single sharp image of the line is replaced by a pair of sharp 

 lines exceedingly close together. On further motion, these 

 diffraction lines slowly separate, becoming fainter until they 

 finally become invisible. Whole revolutions of the micrometer 

 screw are read at " 5," where a portion of the sliding dia- 

 phragm is exposed. The screw itself is divided into tenths, and 

 these tenths constitute the arbitrary units of intensity used. 

 The whole range of the instrument is about four revolutions of 

 the screw, or a range of forty of these units. The probable 

 error of the mean of a series of readings such as was usually 

 taken is found to be somewhat less than one of these units. 



]S T o good work can be done with such instruments without 

 carefully shielding both eyes from extraneous light. The 

 observations were nearly all made on a single line of the 

 aqueous vapor spectrum, the strongest of them all, the line in 

 the red near the " D " line. 



Observations were made regularly at altitudes of 5 degrees,. 

 10 degrees, 20 degrees, 30 degrees, and 90 degrees, for a time 

 at the horizon also. 



Inthe mountain work, the two duplicate instruments were 

 used in simultaneous observations. Although they were of 

 identical construction a comparison was made, the assistant 

 and myself making readings side by side and comparing results. 

 A difference of a very few units was observed, clearly due to a 

 difference of eye-sight. 



The observations described in this paper form a part of a 

 series continued with a few interruptions during several years, 

 most of them being made at the Shattuck Observatory of 

 Dartmouth College, Hanover, New Hampshire. 



Three trips have been made to the White Mountains, with 

 the idea of studying the behavior of the spectrum of aqueous 

 vapor. The first trip resulted in little or nothing of value, the 

 unfavorable nature of the weather, among other reasons, defeat- 

 ing the purpose in view. A second and more successful trial 

 was made in the summer of 1886, and again in 1887, a stay of 

 two weeks being made each summer. The most important 

 object sought was to gain some evidence as to the height to 

 which aqueous vapor extends during stormy weather ; or, more 

 comprehensively, in how far the. spectroscope is competent to 

 give information as to the height of vapor in all kinds of 

 weather. Other questions were also considered, and more or 

 less information bearing on them was gained. 



