LABOURS RELATIVE TO THE SOLAR SYSTEM. 291 
has been so unjustly assigned as the characteristic of the 
illustrious astronomer. 
At the close of 1787, Herschel presented a memoir to 
the Royal Society, the title of which must have made a 
strong impression on people’s imaginations. The author 
therein relates that on the 19th of April, 1787, he had 
observed in the non-illuminated part of the moon, that 
is, in the then dark portion, three volcanoes in a state of 
ignition. Two of these volcanoes appeared to be on the 
decline, the other appeared to be active. Such was then 
Herschel’s conviction of the reality of the phenomenon, 
that the next morning he wrote thus of his first observa- 
tion: “ The voleano burns with more violence than last 
night.” The real diameter of the volcanic light was 
9000 metres (16,400 English feet). Its intensity ap- 
peared very superior to that of the nucleus of a comet 
then in apparition. The observer added: “The objects 
..uated near the crater are feebly illuminated by the 
ight that emanates from it.” Herschel concludes thus: 
“In short, this eruption very much resembles the one I 
witnessed on the 4th of May, 1783.” 
How happens it, after such exact observations, that 
ew astronomers now admit the existence of active vol- 
canoes in the moon? I will explain this singularity in 
a few words. 
The various parts of our satellite are not all equally 
reflecting. Here, it may depend on the form, elsewhere, 
on the nature of the materials. Those persons who have 
examined the moon with telescopes, know how very con- 
siderable the difference arising from these two causes 
may be, how much brighter one point of the moon some- 
times is than those around it. Now, it is quite evident 
that the relations of intensity between the faint parts 
