THE HAMBURG ACCOUNT OF WRIGHT'S THEORY. 183 



which is but increasing a g^o^th part of the great radius 

 of the orbit (which is an error every astronomer will readily 

 grant is very easily made), and we shall find the orbits of 

 the said two comets to be exactly equal. Further, I must 

 inform you that the comet of 1682, which the above 

 [is] compared with, seems to have been so accurately 

 observed that it does not appear to have altered its 

 Perihelion distance half ^g-th part in one entire revolu- 

 tion. Now, if we can with any show of reason, and a 

 probability on our side, bring the areas of these two extreme 

 comets, as I may call them, to an equality, sure we may 

 conclude it is a subject highly worthy to be more considered 

 and enquired into." Thereupon he proceeds to prove that 

 all the light of the planets and of their satellites or moons is 

 borrowed from the sun, they being, in fact, dark bodies. 

 "Now," he says, "to convince you that the planets are all 

 in their own nature no other than dark opaque bodies 

 reflecting only the borrowed light of the sun, I must 

 recommend to your observation this natural and simple 

 experiment, which almost any opportunity of seeing the 

 moon a little before the full, will put into your power to 

 make ; but best and easiest when the sun is in any of the 

 North Signs, i.e. in summer. At such a time the sun, being 

 near setting, the moon will appear in the eastern hemisphere ; 

 and if there be any bright clouds northward or southward 

 near her, you will plainly perceive that the light of the one 

 is of the same nature with that of the other; I mean the 

 light of the moon and that of the cloud. To me there 

 never appeared any difference at all ; and I am persuaded 

 were you to make but two or three observations of this kind, 

 which is from nature itself, a sort of ocular demonstration, 

 you cannot fail of being convinced that the moon's light, 

 such as it is, without heat, can possibly proceed from no 

 other cause than that which illumines the cloud. For, if 

 the clouds whose composition we know to be but a thin 

 light fluid, formed of condensed vapours only, is capable 

 of remitting so great a lustre, how much more may we not 

 allow the moon, which length of time and many other 

 circumstances have long confirmed, to be a durable and 

 solid body." 



THE FOURTH LETTER is entitled : " Of the nature of 

 the Heavenly Bodies continued, with the Opinions of the 



