THE MOON. 



absence of an atmosphere. Be this as it may, however, it is certain 

 that there are no clouds upon the moon, for if there were, we 

 should immediately discover them, "by the variable lights and 

 shadows they would produce. If there be an atmosphere upon tl}e 

 moon, it is therefore one entirely unaccompanied by clouds. 



11. One of the effects produced by a distant view of an atmo- 

 sphere surrounding a globe, one hemisphere of which is illumi- 

 nated by the sun, is, that the boundary, or line of separation, 

 between the hemisphere enlightened by the sun and the dark 

 hemisphere, is not sudden and sharply denned, but is gradual 

 the light fading away by slow degrees into the darkness. It is 

 to this effect upon the globe of the earth that twilight is owing, 

 and as we shall see hereafter, such a gradual fading away of the 

 sun's light is discoverable on some of the planets, upon which an 

 atmosphere is observed. Now, if such an effect of an atmosphere 

 were produced upon the moon, it would be perceived by the naked 

 eye, and still more distinctly with the telescope. When the moon 

 appears as a crescent, its concave edge is the boundary which 

 separates the enlightened from the dark hemisphere. When it is 

 in the quarters, the diameter of the semicircle is also that boun- 

 dary. In neither of these cases, however, do we ever discover 

 any gradual fading away of the light into the darkness ; on the 

 contrary, the boundary, through serrated and irregular, is never- 

 theless perfectly well defined and sudden. All these circumstances- 

 conspire to prove that there does not exist upon the moon an 

 atmosphere capable of refracting light in any sensible degree. 



But it may be contended that an atmosphere may still exist, 

 though too attenuated to produce a sensible twilight. Astro- 

 nomers, however, have resorted to another test of a much more 

 decisive and delicate kind, the nature of which will be understood 



by explaining a simple principle 

 of optics. When a ray of light 

 passes through a transparent 

 medium, such as air, water, or 

 glass, it is generally deflected 

 from its rectilinear course, so as 

 to form an angle. A simple and 

 easily- executed experiment will 

 render this intelligible. Let a 

 visible object, such as a coin, be 

 placed at c, in the bottom of a 

 bucket. Let the eye be placed at E (fig. 3), so that the side of the 

 bucket, when empty, shaU just conceal the coin, and so that the' 

 nearest point to the coin visible shall be at A, in the direction of 

 the line E B A. Let the bucket be now filled with water, and the 



