NO ATMOSPHERE. 



coin will become immediately 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. 



Let us see how this principle can be applied to the case of the 

 moon's atmosphere, if such there be. 



Let m m' (fig. 4) represent the disk of the moon. Let a cf 

 represent the atmosphere which surrounds it. Let s m e and 

 s m' e represent two lines touching the moon at m and m', and 

 proceeding towards the earth. Let s s be two stars seen in the 

 direction of these lines. If the moon had no atmosphere, these 

 stars would appear to touch the edge of the moon at m and m', 

 because the rays of light from them would pass directly 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 pro- 

 Fig. 4. 



*.*' 



perty, stars in such positions as s' &*, behind the edge of the moon, 

 would be visible at the earth, for the ray s' m, s' m ', in passing 

 through the atmosphere, would be bent at an angle in the 

 direction m e', and m' e', so that the stars s' s' would be visible 

 at e' e', notwithstanding the interposition of the edges of the 

 moon. 



This reasoning leads to the conclusion that as the moon moves 

 over the face of the firmament, stars will be continually visible 

 at its edge which are really behind it if it have an atmosphere, 

 and the extent to which this effect will take place will be in pro- 

 portion to the density of the atmosphere. 



12. The magnitude and motion of the moon and the relative 

 positions of the stars are so accurately known that nothing is 

 more easy, certain, and precise, than the observations which may 

 be made with the view of ascertaining whether any stars are ever 

 seen which are sensibly behind the edge of the moon. Such 

 observations have been made, and no such effect has ever been 

 detected. This species of observation is susceptible of such extreme 

 accuracy, that it is certain that if an atmosphere existed upon the 



39 



