NO. 8 SAMUEL PIERPONT LANGLEY — ABBOT 21 



the wave-lengths of extreme infra-red rays " give us our first knowl- 

 edge of this terra incognita. I say ' knowledge,' with the admission 

 that this knowledge is as yet alloyed with those imperfections which 

 are inherent in the most painstaking work in an utterly new field. All 

 here is so new that the difficulties themselves are of a quite unfamiliar 

 kind ; for it is well to bear in mind that though all our observations, 

 from first to last, are made on an amount of heat which may be well 

 called infinitesimal, it is still the kind of radiations which produce 

 this heat rather than the amount which forms the greatest difficulty. 

 This, as we shall see, is because this heat seems to be largely that 

 absorbed and reradiated from the substance of the lunar soil, and 

 whose temperature is consequently so low as to be in constant danger 

 of being confused with the heat from the terrestrial media it has 

 passed and from the different parts of the apparatus itself — a difficulty 

 which, when the thing in question is to ordinary sense both invisible 

 and inappreciable, constitutes an obstacle almost insurmountable, when 

 we design to go beyond those features which Lord Rosse succeeded 

 in noting. We notice, in particular, that however successfully we may 

 protect our apparatus from the radiations of surrounding objects, we 

 must always, in the nature of the case, either actually or virtually, 

 interpose a screen at intervals to interrupt the heat we are measuring. 

 In ordinary spectrothermal work, as in that on the sun, the radiations 

 of this screen are perfectly negligible, and would be so if the sun's heat, 

 while the same in kind as now, were no greater in amount than the 

 moon's. Here, on the contrary, because they are of the same kind as 

 those radiated from the moon's cold surface, they become of the first 

 importance, so that a special study of the radiation of the screen 

 becomes a necessity. 



" There are three princi})al methods of investigation : First, the 

 measurement of the total heat of the moon with a concave mirror of 

 short focus, concentrating it as much as possible and admitting the 

 interposition of a sheet of glass to rudely indicate the quality of lunar 

 rays as compared with those of the sun. This method, which was that 

 employed by Lord Rosse, has been very thoroughly practiced here with 

 results which have been partly given in the previous memoir. The sec- 

 ond method has been to form, usually with this same mirror, an image 

 of the moon, but this now falls upon the slit of a special spectroscope 



"See Am. Journ. of Sci., XXXII, August, 1886, 'On Hitherto Unrecognized 

 Wave-Lengths ' ; also an article in Annales de Qiim. et de Phys., 6 ser. T. IX 

 December, 1886, ' Sur les spectres invisibles.' 



