36 Prof. 8. P. Langley on the 



time), we have taken advantage of the sensitiveness of our 

 apparatus to directly explore a large lunar image with the 

 bolometer, in spite of the diminished heat in such a one. For 

 this purpose a special mirror, 303 millim. in diameter and 

 3137 millim. focus, giving a lunar image of about 30 millim. 

 diameter, has been employed. On the occasion of a lunar 

 eclipse the last-named apparatus has also been used. 



We have already alluded (see Memoirs of the National 

 Academy, vol. iii. p. 20) to the especial importance of the 

 action of the screen in these observations. This arises from 

 the fact, as will be seen later, that we shall deal with lunar 

 heat of a totally different quality from that of moonlight or 

 sunlight. It is, in a large part, radiation emanating from the 

 lunar soil, and of a quality, as we shall see, approximating 

 to that of the screen itself. It must be evident, then, that the 

 radiations from this screen assume here a wholly abnormal 

 importance. 



An investigation of the theory of the screen accordingly 

 occupies a chapter, for which the reader is referred to the 

 original. We may remark here, in passing, that the investi- 

 gation incidentally offers an explanation of the empirically 

 known fact, that the velocity of cooling of a hot body at various 

 temperatures of excess varies with that of the enclosure itself. 

 The discussion also indicates what appears to be an inde- 

 pendent method of determining the absolute zero ; but the 

 method, although apparently correct in theory, w r ould demand 

 observations more accurate than our own casual ones to give 

 it practical value. It may be, however, worth mentioning 

 that the observations, such as they are, indicate by this novel 

 method the existence of an absolute zero at a point between 

 -250° and -300° Cent. 



A list of all the observations in the lunar spectrum extend- 

 ing from October 1884 to February 1887 is then given, 

 together with some collateral ones upon the " great radiator." 

 This latter instrument may be briefly described as analogous 

 to an immense Leslie cube, presenting as it does a blackened 

 radiating surface at the temperature of boiling water and of 

 1 square metre area. The object in giving it this extraordi- 

 nary dimension is to enable it to still angularly subtend the 

 whole field of view of the bolometer, while it is at such a dis- 

 tance that the intervening column of air may be supposed to 

 exercise a measurable absorptive effect. Its actual distance 

 was 100 metres, and at this distance the absorption of the 

 intervening air on the dark radiant heat, emanating from its 

 surface at 100° Cent., was in fact manifest, and gave evidence, 

 novel and interesting, both as to the actual absorption by our 



