138 



INFRA-RED EMISSION SPECTRA. 



expected from the full moon. Boeddicker 1 also records that the minimum 

 of the heat-effect falls decidedly later than the minimum illumination. 



350 

 Abs 



20 



300 

 Abs 



280 



REFLECTION AND RADIATION FROM THE MOON. 



Using the data just computed, we are now in a position to make a 

 rough comparison of the relative amounts of energy reflected by and radi- 

 ated from the moon. In 

 the short wave-lengths the 

 reflecting power depends 

 upon the refractive index, 

 while at 8 to io n the re- 

 flecting power will depend 

 upon the refractive index 

 and the extinction coefficient 

 (see Drude's Optics, p. 336), 

 and at all times will have a 

 high value, except at 7 a. 

 The reflecting power of the 

 moon is variously recorded 2 

 from 0.09 to 0.23, which val- 

 ues are several times higher 

 than the refractive index of 

 ordinary rocks permits. 

 This seems to indicate in- 

 ternal reflection. 



The question is therefore 

 reduced to finding the ratio 

 of the energy emitted by 

 the moon to the energy of 

 the sun, reflected from the 

 lunar surface. For this pur- 

 pose the temperatures of the 

 moon and of the sun (350 

 abs. and 5900 abs., respect- 

 ively) given on the preceding 

 pages are employed. The 

 values are taken slightly lower, although it makes but slight difference in 

 the final computation. We are not so much concerned with the question 

 as to where the maximum emission of the moon occurs as we are in the 

 fact that, in the region of 8 to 10 [x, where Langley observed radiation from 

 the moon, there is also reflected energy from the sun. From the transmis- 



260 



240 



220 



200 



20 40 60 80 100 120 i40mcnu{es 



Fig. s>7. Lunar temperature decadence during eclipse. 



1 Boeddicker: Trans. Roy. Dublin Soc, 3, p. 321, 1885. 



2 Very: Astrophysical Jour., 8, p. 276, etc., 1898. 



