S. P. Langley — Temperature of the Moon. 425 



that the so called temperature of space (a term clue to Fourier 

 and afterwards adopted by Pouillet who fixed its value at 

 — 142° Cent.) has no sensible existence, but ma}' be here treated 

 as that of the absolute zero. 



There is next given a description of the apparatus, which 

 consists essentially of a siderostat, carrying an 18-inch mirror, 

 and capable of sending a lunar beam of corresponding capacity 

 horizontally into an adjoining dark room and keeping it fixed 

 there during the night's observations. In the path of the 

 beam can be interposed a large double screen of blackened cop- 

 per, ordinarily filled with water. The beam then falls on a 

 condensing mirror, whose ordinary aperture of 8 inches does 

 not, as may be seen, utilize the whole of the beam transmitted 

 by the siderostat, but has been selected in reference to the 

 capacity of the rock-salt train of lenses and prisms which forms 

 the spectrum. This train is believed to consist of pieces of 

 salt of hitherto unapproached perfection in workmanship, as at 

 the time our investigation commenced, no salt prisms were pro- 

 curable giving a single Fraunhofer line in the solar spectrum ; 

 while with the actual rock-salt train, D is divided in the 

 spectrum of the moon. The general construction of the spec- 

 trobolometer, and of the special bolometer employed with it, 

 will be found given in previous papers.* 



There are three principal methods of investigation : 



First; the measurement of the total heat of the moon with 

 a concave mirror, admitting the interposition of a sheet of 

 glass to rudely indicate the quality of lunar rays as com- 

 pared with those of the sun. This method, which was that 

 employed by Lord Rosse, has been very thoroughly practised 

 here with results which have been partly given in the previous 

 memoir. 



Second', A method, practised here for the first time, and 

 yielding quite peculiar results, has been to form, usually with 

 this same mirror, an image of the moon, but to now let this 

 fall upon the slit of a special spectroscope provided with the 

 rock-salt train referred to ; and after expanding this excessively 

 minute heat in this way, it has been found possible, with late 

 improvements in the apparatus, to measure by the bolometer 

 the different degrees of heat in the different parts of this lunar 

 heat-spectrum, both visible and invisible. The doing of this, 

 with its results, forms the principal subject of the present 

 memoir. 



Third', since such a mirror as that just mentioned, owing to 

 its short focus, forms an extremely small lunar image, in cer- 



* See this Journal, xxv, March, 1883, and xxxi, January, 1886. For a 

 ■description of the improved form of bolometer and galvanometer, see vol. xxxii, 

 August, 1886. 



