ters, the diameter of the semi-circle is also that boundary. In neither of these 

 cases, however, do we ever discover the slightest indication of any such ap- 

 pearance as that which has just been described. There is no gradual fading 

 away of the light into the darkness ; on the contrary, the boundary, though 

 serrated and irregular, is nevertheless perfectly well-defined and sudden. 



All these circumstances conspire to raise a presumption that there does not 

 exist upon the moon any atmosphere capable of reflecting light in any sensible 

 degree. 



But it may be contended that an atmosphere may still exist, though too atten- 

 uated to produce a sensible twilight. Astronomers, 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 cent-piece, be placed at C, in the bottom of a 

 bucket. Let the eye be placed at E, so that the side of the bucket, when 

 empty, shall just conceal the coin from the eye, and so that the nearest point to 

 the coin visible to the eye shall be at A, in the direction of the line E B A. 

 Let the bucket be now filled with water, and the coin will become immediate- 

 ly visible ; the reason of which is, that the ray of light C B proceeding from the 

 coin is bent at an angle in passing from the water into the air, and reaches the eye 

 by the angular course C B E. Thus it appears that the coin will be visible 

 to the eye, notwithstanding the interposition of the opaque side of the bucket. 



Fig. 3. 



Let us see how this principle can be applied to the case of the moon's atmo- 

 sphere, if such there be. Let MN (fig. 4) represent the disk of the moon. Let AB 

 represent the atmosphere which surrounds it. Let C D and E F represent two 

 lines touching the moon at M and N, and proceeding toward the earth. Let 

 S T be two stars seen in the direction of these lines. If the moon had no at- 

 mosphere, these stars would appear to touch the edge of the moon at M and 

 N, because the rays of light from them would pass directly along the lines 

 S M D and T N F toward the earth ; but if the moon have an atmosphere, then 

 that atmosphere will possess the property which is common to all transparent 

 media of refracting light, and, in virtue of such property, stars in such positions as 

 Q and R, behind the edge of the moon, would be visible at the earth, for the ray 

 Q M, in passing through the atmosphere, would be bent at an angle in the direction 

 Q M P, and in like manner the ray R N would be bent at the angle R N so that 

 the stars Q and R would be visible at P and O, notwithstanding the interposi- 

 tion of the edges of the moon. This effect is precisely the same as that in the 

 example of the coin in the bucket ; the ray from the star is bent over the edge 

 of the moon so as to render the star visible notwithstanding the interposition of 



