LUNAR ECLIPSES. 



render the boundary of the latter so distinct, that the phenomenon 

 presents one of the most striking evidences of the rotundity of the 

 earth, the form of the shadow being accurately that which one 

 globe would project upon another. 



45. If the earth were not surrounded with an atmosphere 

 capable of refracting the sun's light, the disc of the moon would 

 be absolutely invisible after entering within the 

 edge of the shadow. For the same reason, how- 

 ever, that we continue to see the sun's disc, and 

 receive its rays after it has really descended 

 below the horizon, an observer placed upon the 

 moon, and therefore the surface of the moon 

 itself, must continue to receive the sun's rays 

 after the interposition of the edge of the earth's 

 disc as seen from the moon. This refracted light 

 falling upon the moon after it has entered within 

 the limits of the shadow, produces upon it a 

 peculiar illumination, corresponding in faintness 

 and colour to the rays thus transmitted through 

 the earth's atmosphere. 



To render this more clear, let e e', fig. 20, 

 represent a diameter of the earth at right angles 

 to the axis cf of the shadow, and let a a' repre- 

 sent the limits of the atmosphere. Let s e f be 

 the ray proceeding from the edge of the sun, and 

 forming therefore the boundary of the shadow, 

 considered without reference to the atmosphere. 

 But the solar rays in passing through the convex 

 shell of air, between a and e, are affected as 

 they would be by a convex lens composed of a 

 transparent refracting medium, and are therefore 

 rendered convergent, so that the ray s e, instead 

 of passing directly to m, will be bent inwards 

 towards m', while the ray which really passes 

 from e to / is one which comes in the direction 

 s' e, and therefore from a point within the 

 sun's disc. The moon's disc, therefore, or any point of it which 

 is within the angle m e m', will receive this refracted light, 

 and will be illuminated by it in accordance with its colour and 

 intensity. 



The deflection which a solar ray suffers in passing through the 

 atmosphere towards e on the side of the sun, is equal to the hori- 

 zontal refraction ; and as, according to the principles of optics, 

 it suffers an equal refraction in passing out on the other side, the 

 total deflection, which is measured by the angle m e m' t is twice 



183 



