NO. 8 SAMUEL PIERPONT LANGLEY ABBOT 2$ 



absorption in the summer. This increase would be still more marked 

 if only the coldest and driest days of winter had been compared with 

 the most humid of summer 



". . , . The most reliable spectrum comparisons with a blackened 

 screen show an average * etTective lunar temperature' of +45. °C. 

 near the time of full moon. 



". . . . A measurement .... gives for the ratio of 

 reflected radiation _ 

 emitted radiation 

 This, it is to be remembered, is the ratio after absorption by the 

 earth's atmosphere ; but the extreme infra-red rays may have suf- 

 fered unduly in passing this barrier " 



These researches on the temperature and spectrum of the moon 

 entailed observations at Allegheny on more than 50 nights spread 

 over the coldest of winter and the hottest of summer, as well as in 

 months less trying to the observers, from October 1884 to February 

 1887. The spectrum observations alone, absolutely pioneering in 

 character, of which only mean values are quoted here, occupied 22 

 nights, besides the preparation for them on uncounted days. 



In order to avoid errors from the scattering of the more abundant 

 rays of other wave lengths into the weaker regions observed in the 

 lunar spectrum, Langley was obliged to use two spectroscopes in 

 tandem, each employing a rock-salt prism because glass is opaque to 

 such rays as are emitted by cool bodies like the moon. The common 

 experience of the salt shaker at the dinner table has taught us how 

 readily rock salt absorbs water. The slightest cloud of mist upon a 

 rock-salt prism is prejudicial to its optical performance. It is easy to 

 imagine, therefore, how often in summer the spectral observations of 

 the moon were interrupted, and Dr. Langley's good friend Mr. 

 Brashear came to the rescue by resurfacing the prisms. 



"ON HITHERTO UNRECOGNIZED WAVE-LENGTHS" 



" We are led to take this labor, not primarily to settle the theo- 

 retical questions involved in determining the relation between dis- 

 persion and wave-length (though these are most interesting), but 

 with the object of providing a way which will hereafter enable any 

 observer to determine the visible or invisible wave-lengths of any 

 heat, whether from a celestial or terrestrial source, observed in any 

 prism ; and thus to gain that knowledge of the intimate constitution 

 of radiant bodies which an acquaintance with the vibratory period of 

 their molecules can usually alone afford us. It is this considerable 



