374 PHILOSOPHICAL TRANSACTIONS. [aNNO 173Q. 



proves the existence of a lunar atmosphere, as will be plain to any one who 

 hides the sun froni him by balls of wood, or other opaque matter. Hence this 

 is not to be ascribed to a lunar, but to a solar atmosphere; as has been proved 

 by M. Mairan, in his treatise on the Aurora Borealis, p. 14, i. 



3dly. The diminution of the lunar diameter, which in solar eclipses is ob- 

 served to be about 30' less than when the moon shines with a full orb, in the 

 same degree of anomaly, by no means proves a lunar atmosphere; though some 

 inequalities of mountains are observed in the circumference of the moon's disk, 

 which quite disappear in the full moon ; for lucid objects strike the fibres of the 

 eye so strongly, that their motion is communicated to the neighbouring fibres, 

 and so the image of the lucid body is increased beyond the due quantity, as is 

 known by common experience : for if a stick be placed between the eye and 

 the moon, the diameter of the interposed stick will seem to be diminished; but 

 if at that time any cloud pass before the moon, the diameter of the stick will 

 appear less diminished; and if the moon be quite obscured, there will be no 

 diminution at all; and lastly, the diminution will vary according to the various 

 intensity of the moon's light. 



As for the inequalities of the mountains, they are, for the same reason, ob- 

 served least in the full moon; for the lunar mountains, obscure of themselves, 

 and seen in the bright orb of the sun, escape the eye much less, than when, 

 shining in the full moon, they are extinguished in the splendour of that lumi- 

 nary ; especially as the lunar light is so intense, that a star of the 3d magnitude 

 can hardly be seen, when near it. But, to remove all doubt, if the lin)b of 

 the moon, when in opposition to the sun, were the bound of an atmosphere, 

 and not of her very body itself, the mountains in her circumference would 

 never be observed by the longer telescopes with narrow objective apertures. 

 Whereas M. Fouchy had often observed several inequalities of mountains in 

 the disk of the full moon, with a telescope of 3d Paris feet, and an objective 

 aperture of 1 inch : hence it follows that the disk of the full moon is bounded 

 by the periphery of her body, and not of her atmosphere. 



4thly. We must not omit to speak of that remarkable observation, in 1715, 

 of the lunar corusaitions, made by M. Delouville, in the presence of many 

 astronomers of the Royal Society, and seen by Dr. Halley, N° 343, vol. vi, p. 

 158 of these Abridgments. We may suppose that the moon's visible limb is 

 composed of the tops of mountains; which, in a total eclipse, hide the sun 

 from an observer, in the same manner as the trees in great woods obstruct the 

 sight. Hence if some rows of mountains, on the moon's surface, afford a 

 free and direct passage for the solar rays, they must imitate a kind of corusca- 

 tions, like as when, i« a camera obscura, a ray of the sun is suddenly admitted 



