lO 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 68 



path through the spectroscope making a total path of 128.5 nieters 

 (422 feet). The second arrangement totaled 245.5 meters (806 

 feet). The source N could be turned towards the slit and its radia- 

 tion passed directly into the spectroscope. Its path through water 

 vapor then amounted to only 11.5 meters (38 feet). 



The differences of the observed intensities of the spectrum energy 

 after passing over the various paths served to measure the energy 

 absorbed in the water vapor. 



In consequence of the long path of aliout 43 meters to the colli- 

 mator M^, but a small part of the energy sent out by the lamp was 

 utilized. A shorter focus collimator would have been no better 

 except with a point source of radiation ; for as the source was 10 cm. 

 tall, the extreme divergence of the bundles of parallel rays from the 



Fig. 3. — F, principal focus of lens ; b, colHmated rays from point source ; 

 C, colHmated rays from linear source. 



two ends of the source would have overbalanced the gain in incident 

 energy on the collimator from the lamp (see fig. 3). Unfortunately 

 there was a serious vibration of the image of the lamp formed on 

 the slit due to the great magnification of the small tremors of the 

 mirrors. These two causes, the small angular aperture of the mirror 

 and tremor of the image, went far to limit the accuracy obtainable 

 in the observations. 



The energy, after entering the slit of the spectroscope, passed to 

 a cylindrical collimator (straight elements vertical) of 543 cm. 

 focus, diaphragmed down to 12 by 9 cm. ; thence it passed to a rock- 

 salt prism with Wadsworth minimum-deviation attachment, then to 

 the concave mirror of 75 cm. focus which formed the spectrum on 

 the bolometer. All the mirrors were silvered on glass. 



THE WATER VAPOR 

 The water vapor was contained in a large double-walled galvan- 

 ized-iron tube, open at its ends. Suitable compartments being pro- 

 vided in the double wall, and numerous holes leading from these 



