292 HERSCHEL. 



and the brilliant parts must continue to exist, whatever 

 be the origin of the illuminating light. In the portion 

 of the lunar globe that is illuminated by the sun, there 

 are, everybody knows, some points, the brightness of 

 which is extraordinary compared to those around them ; 

 those, same points, when they are seen in that portion of 

 the moon that is only lighted by the earth, or in the ash- 

 coloured part, will still predominate over the neighbour- 

 ing regions by their comparative intensity. Thus we 

 may explain the observations of the Slough astronomer, 

 without recurring to volcanoes. Whilst the great ob- 

 server was studying in the non-illuminated portion of 

 the moon, the supposed volcano of the 20th of April, 

 1787, his nine-foot telescope showed him in truth, by 

 the aid of the secondary rays proceeding from the earth, 

 even the darkest spots. 



Herschel did not recur to the discussion of the sup- 

 posed actually burning lunar volcanoes, until 1791. In 

 the volume of the Philosophical Transactions for 1792, 

 he relates that, in directing a twenty-foot telescope, 

 magnifying 360 times, to the entirely eclipsed moon on 

 the 22d of October, 1790, there were visible, over the 

 whole face of the satellite, about a hundred and fifty 

 very luminous red points. The author declares that he 

 will observe the greatest reserve relative to the simi- 

 larity of all these points, their great brightness, and 

 their remarkable colour. 



Yet is not red the usual colour of the moon when 

 eclipsed, and when it has not entirely disappeared ? 

 Could the solar rays reaching our satellite by the effect 

 of refraction, and after an absorption experienced in the 

 lowest strata of the terrestrial atmosphere, receive an- 

 other tint ? Are there not in the moon, when freely 



