216 HERSCHEL. 



tlie tlieu dark iiortion — tLree volcanoes in a state of ignition. Two of 

 these volcanoes appeared to be on the decline, the other appeared to be ac- 

 tive. Such was his conviction of the reality of the phenomenon, that the 

 next morning he made the following record : '^ The volcano burns with 

 more violence than last night." The real diameter of the volcanic light 

 was 5,000 metei's, (10,400 English feet.) Its intensitj' appeared very 

 superior to that of the nucleus of a comet then in api:)arition. He 

 further added : " The objects situated near the crater are feebly illumin- 

 ated by the light that emanates from it ;" and concludes thus : " In short, 

 this eruption very much resembles the one I witnessed on the 4th of 

 May, 1783." 



How liappens it, after such exact observations, that few astronomers 

 now admit the existence of active volcanoes in the moon ? I will explain 

 this singularity in a few words. 



The various parts of our satellite do not all equally reflect light. Here, 

 it may depend on the form, elsewhere, on the nature of the materials. 

 Those who have examined the moon with telescopes know how very 

 considerable the difference arising from these two causes may be, how 

 much brighter one point of the moon sometimes is than those around 

 it. Now, it is quite evident that the relations of intensity between the 

 faint parts and the brilliant ones 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, everyone knows, some points, 

 the brightness of which is extraordinary compared to those around them ; 

 those same points, when they are seen in that i^ortion of the moon that is 

 only lighted by the earth, or in the ash-colored part, will still predominate 

 over the neighboring regions by their comparative intensity. Thus we 

 may explain the observations of the Slough astronomer, without recur- 

 ring to volcanoes. While the observer was studying in the non-illumin- 

 ated 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 supposed 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 three hundred and sixty 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 relativ^e to the 

 nature of all these points, their great brightness, and their remarkable 

 color. 



Yet is not red the usual color 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 any other tint"? 

 Are there not in the moon, when freely illuminated, and opposite to the 



