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



Akt. XXXII. — A Mountain Study of the Spectrum of 

 Aqueous Vapor / by Chaeles S. Cook. 



A complete study of the Solar Spectrum necessarily in- 

 cludes, not only a determination of the position of all iines r 

 but also an estimation of their relative intensities. While the 

 means of determining the position of lines leaves little to be 

 desired, it is a remarkable fact that no satisfactory method of 

 measuring the intensity of the absorption lines has been de- 

 vised. The instrument devised by the writer for the study of 

 the variations of some of the terrestrial lines has done excel- 

 lent service, and it is possible that it might be adapted to more 

 general use in the study of the spectrum. While observations 

 have been made on a number of the atmospheric lines due to 

 dry air, the study of the aqueous vapor lines has proven most 

 interesting. Two spectroscopes of identical construction were 

 made especially for this work. They were provided by the 

 United States Signal Service. 



The first requisite in such an instrument is that it shall give 

 a bright spectrum. As observations must be made on dark, 

 stormy days, every effort must be made to secure a strong 

 illumination. This is attained by the use of a large collimat- 

 ing lens and prism, and a large angular aperture for the colli- 

 mator* which makes it best that the collimating tube be short. 

 Only moderate magnifying power in the observing telescope 

 is desired, as a high power would make the spectrum appear 

 faint. 



Another requirement of a general nature is that such a 

 spectroscope shall be so designed that it is not liable to be 

 accidentally thrown out of adjustment ; for a slight change in 

 the focus or in the width of the slit, while it might not be 

 detected, would make a great error in the photometric value of 

 the spectrum lines. For this reason, each draw-tube should be 

 provided with a set screw, and the slit adjustment should be 

 controlled by parts which are inclosed within the cap at the 

 end of the collimator. Reference is not made here to a brass 

 cover which must be removed when the instrument is used,, 

 but rather to a cap with a plane glass front which is never to 

 be removed during the course of a series of observations. 

 This last requirement, unfortunately, is not met in the spectro- 

 scope as constructed for use, as the maker violated his instruc- 

 tions in that particular, as he did not find it convenient to 

 inclose the adjusting collar d in the cap. Fig. 1. 



The instrument has an altazimuth movement, and is pro- 

 vided with leveling screws and a spirit level, which last is 

 fastened to the top of the prism case. A graduated arc shows 



