142 I'KOCEEDINGS OF THE AMERICAN ACADEMY 



1888, to make as complete an investigation as possible of these and 

 allied matters. With this purpose in view, two complete sets of 

 apparatus were prepared, and two parties of observers were in the 

 field. Upon the night of the eclipse, hov/ever, it rained, and observa- 

 tion was consequently impossible. 



If we are to conclude that our atmosphere is opaque to radiations 

 from the lunar rocks, we must endeavor to explain the observed be- 

 havior of the lunar rays, with reference to absorbing media, upon the 

 hypothesis of selective reflection. We have found that 31^^ of the 

 lunar rays will pass through quartz ; and if it is observed that ordi- 

 nary rocks absorb and reflect selectively, taking up the short waves, 

 and reflecting the long, the absorption of the lunar rays may be as 

 effectively explained as upon the supposition that we receive radiations 

 from the lunar rocks. 



We have arranged to reflect the solar rays from rocks, and, by 

 y)lacing in the path of the reflected ray the same quartz plate that was 

 used in the lunar observations, have ascertained the amount of absorp- 

 tion. The results ai'e shown in tabular form below. 



Matpriql Mean Dffleotioti Mean Deflection Per Cent 



without Qutirtz. through Quartz. transmitted. 



Red Lava, Porous .... 153 112 73 



Quartz 131 116 88 



Black Lava 82 65 80 



White Marble 180 147 82 



Slate 103 77.8 79 



Direct rays of sun transmitted .... .... 93 



The results show that selective absorption takes place, but to an 

 extent quite limited, and altogether insufficient to explain the great 

 absorption of the lunar rays. When we consider the great differences 

 in character of the specimens of rocks experimented upon, as compared 

 with the very inconsiderable differences in the transmitted rays, it 

 seems useless to undertake to find a material that shall absorb so large 

 a proportion of the short solar waves that only one third of the re- 

 flected rays will pass through the quartz plate. 



The question is how to reconcile the results here obtained with the 

 eclipse observations, showing that our atmosphere is nearly opaque to 

 radiations from the lunar rocks. The materials for the solution of 

 this problem do not seem to be at hand. It is, of course, possible that 

 the materials of the moon's surface may absorb selectively to a much 

 greater extent than those that we have examined. On the other 

 hand, if the lunar surface were of very light and porous materials, 



