THE ILLUMINATION OF THE ECLIPSED MOON. 



357 



tho lower, and especially the lowest, strata of the earth's 

 atmosphere; secondly, that in consequence of the amount 

 of vapour in these low strata, the blue rays are largely 

 absorbed, and tho I'ed rays chiefly transmitted ; thirdly, 

 that it is these red rays that give to the eclipsed moon its 

 ruddy tint ; and fourthly, that the degree of illumination 

 of the eclipsed moon is de})endont on tlio meteorological 

 conditions of the atmospheric ring through which the sun's 

 rays pass to reach the moon. Up to this point there seems to 

 be a general agreement, but as to the particular meteoro- 

 logical conditions which favour a light eclipse or a dark 

 eclipse, there is a divergence of opinion, some holding that 

 the dark eclipses occur when the atmosphere is unusually 

 dry, others wlion it is unusually humid, while Sir John 

 Herschel attributed the variation to the varying amount 

 of cloud. 



The lirst of tho above propositions — that the illumination 

 is due to the refraction of tho sun's rays through the 

 lower strata of the atmosphere — may be admitted with two 

 (lualifications. In tho first place, I do not think that the 

 whole of tlio light is derived from tho sun's refracted rays, 

 although the greater part may bo ; and in the second place, 

 I believe that tho higher regions of the atmosphere, up 

 to a certain level, are quite as much concerned as tho lower 

 regions. I shall have to refer to this point again. 



That the ruddy tint of the eclipsed moon is due to the 

 vapour in the air absorbing the bluo rays and transuutting 

 specially tho rod rays, is a view wliich I think open 

 to question, and also unnecessary. Why should not the 

 ruddy tint bo the natural colour of the moon ? It can 

 scarcely 1)0 supposed that tho moon has no colour. True 

 wo see none under ordinary circumstances, but then the 

 colour may bo extinguished by the excessive brilliancy of 



