290 HERSCHEL. 
Sir David Brewster, appreciates this view of Herschel’s: 
“It is not conceivable that luminous clouds, ceding to 
the lightest impulses and in a state of constant change, 
can be the source of the sun’s devouring flame and of 
the dazzling light which it emits; nor can we admit 
besides, that the feeble barrier formed by planetary 
clouds would shelter the objects that it might cover, 
from the destructive effects of the superior elements.” 
Sir D. Brewster imagines that the non-luminous rays 
of caloric, which form a constituent part of the solar 
light, are emitted by the dark nucleus of the sun; whilst 
the visible coloured rays proceed from the luminous 
matter by which the nucleus is surrounded. “ From 
thence,” he says, “ proceeds the reason of light and heat 
always appearing in a state of combination: the one 
emanation cannot be obtained without the other. With 
this hypothesis we should explain naturally why it is 
hottest when there are most spots, because the heat of 
the nucleus would then reach us without having been 
weakened by the atmosphere that it usually has to tra- 
verse.” But it is far from being an ascertained fact, 
that we experience increased heat during the apparition 
of solar spots; the inverse phenomenon is more prob- 
ably true. 
Herschel occupied himself also with the physical con- 
stitution of the moon. In 1780, he sought to measure 
the height of our satellite’s mountains. The conclusion 
that he drew from his observations was, that few of the 
lunar mountains exceed 800 metres (or 2600 feet). 
More recent selenographic studies differ from this con- 
clusion. There is reason to observe on this occasion 
how much the result surmised by Herschel differs from 
any tendency to the extraordinary or the gigantic, that 
