VOL. XLI.] PHILOSOPHICAL TRANSACTION-S. 375 



bv means of a speculum, and the picture of the external objects, drawn on the 

 focus of the lens, is taken away, it will be enlightened with luminous traces, 

 much resembling lightning. 



From all tliese it is manifest, that there is nothing like a lunar atmosphere in 

 solar eclipses. We may now speak of the occupations of the stars and planets 

 by the moon. 



If the moon be surrounded by an atmosphere, the planets and fixed stars 

 must be seen, by an observer on the earth, to immerge behind the moon later, 

 and to emerge from it sooner, than if she have no atmosphere; and even in 

 some plnces where such occultations ought to appear, there would be none. To 

 make this plain, let abc, fig. 5, be the moon's body, and s a star at immense, 

 or as at an infinite distance; then the parallel rays lv, mx, touching the moon's 

 body on all sides, inclose a cylindric space, of which the base vzx includes all 

 the parts of the earth's surface to which the star or planet is occulted by the 

 moon. The observer therefore will see the beginning of the eclipse at v, and 

 the end at x, and will measure the duration of time in which the moon may 

 run through her diameter, or an equal space. But if we suppose an atmos- 

 phere of the moon, the ray iw will not remain parallel to the axis of the 

 cylinder, but this will now become a cone, of which the section ytu will mark 

 the earth's surface where the eclipse will be seen. Now the base ytu being 

 contracted, the point y wilj come on any part later than the point v, and the 

 point u will quit it sooner than x; therefore the occultation will begin later, 

 and end sooner, by supposing an atmosphere about the moon, than the con- 

 trary; and there will be no occultation in some places, where it ought to be 

 observed without an atmosphere; for the place c, being included in the base 

 vzx of the former cylinder, is without the conical section ytu. Besides, sup- 

 posing the horizontal refraction in the lunar atmosphere equal to 8'', then vy 

 will be 1384 toises, or a quarter of a Paris league; and hence it follows, that 

 no eclipse must be observed in places according to calculation, whenever they 

 are without a circle of a quarter of a league radius. 



Another phenomenon also takes place on the supposition of a lunar atmos- 

 phere: in the part of the cylinder yr the star indeed will always be seen, but it 

 will be through that medium : hence it will acquire a motion and colour different 

 from the genuine, and that in all eclipses whatever, whether the star be one of 

 the largest or smallest. 



Besides, the duration of such occultations does not seem at all diminished, 

 but is always found to be exactly agreeable to the moon's diameter and motion. 

 As to those observations in which the star, after the contact, is seen to proceed 

 a little in the moon's disk before the occultation, the whole cause of them may 



