22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



provided with a train of rock-salt lenses and a salt prism of excep- 

 tional size and purity ; 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 spectrum ; and 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 certain 

 observations, carried on, however, only during a limited time, we have 

 taken advantage of the sensitiveness of our apparatus to 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 mm in diameter 

 and 3,137 mm focus, giving a lunar image of about 30 mm diameter, 

 has been employed.' On the special occasion of a lunar eclipse the 

 last-named apparatus has also been used. 



" Let it be remembered that every observation on radiant heat, how- 

 ever conducted, whether by the thermometer, the bolometer or thermo- 

 pile, on the sun or moon, or on a neighboring candle — every observa- 

 tion in radiant heat, we repeat, involves the use of a screen at some 

 stage in the process ; since its use is inherent from the very nature of 

 the observation. Again, let it be remembered that, in this peculiar 

 case, the screen itself not only intercepts other rays, but contributes 

 radiations of its own of like quality and amount to those which we 

 would study, and the importance of the investigation to be shortly 

 given on its theory becomes manifest. It will be seen later that the 

 screen is used as little as possible, and that to this end every observa- 

 tion on the moon is preceded by one on the adjacent sky to the east 

 and followed by one on the adjacent sky to the west ; and that the 

 lunar radiation is compared in every case immediately with the mean 

 of the last two and only mediately with that of the screen, whose use 

 we might here appear to be able to dispense with, but which is in fact 

 imposed upon us, we repeat, at some time in the course of the observa- 

 tions by conditions inherent in the nature of the observations 

 themselves. 



" The conclusion of the whole matter is. that we have been dealing 

 with a subject almost on the limit of our power of investigation with 



' This special mirror has been kindly loaned to us by Mr. J. A. Brashear, of 

 Allegheny. 



