28 PROCEEDINGS OF THE AMERICAN ACADEMY 



contact. In tlie interval between the exterior and interior contacts, 

 the edge of the planet has sometimes been traced beyond the limb of 

 the sun. The effect was noticed by Mr. Searle shortly after the third 

 contact, as above stated. According to his recollection of the appear- 

 ance of the field, the difference in darkness beween the planet's disk 

 and the sky was obvious, and might have been expected to make the 

 planet more distinctly visible outside of the sun's limb than was 

 actually the case. 



No appreciable light could be received from Venus itself, unless that 

 planet is incandescent or phosphorescent, an extremely improbable 

 hypothesis. Doubtless the greater portion of the light, like that of 

 the sky near the sun, is due to reflection of the light of the sun from 

 the particles of the earth's atmosphere. During a total solar eclipse 

 the interposition of the moon suffices to cut off nearly all the light 

 near the sun, except the small portion due to the solar corona. It is 

 therefore obvious that the light of the sky near the sun originates at 

 no great distance from the earth, and is doubtless caused by reflection 

 in the terrestrial atmosphere. In a communication to this Academy nine 

 years ago {Proceedings, IX. 1), I showed that many of the phenomena 

 of atmospheric illumination and polarization could be explained by 

 specular reflection from the particles of the air, whose index of refrac- 

 tion differs very slightly from unity. In this case, if the sun was 

 reduced to a point, the light of the sky at small distances would vary 

 inversely as the fourth power of the distance. In any case, a glance 

 towards the sun is sufficient to show that the light increases very rap- 

 idly as we approach the sun's limb. We should expect that the light 

 of the portion of the atmosphere between us and the sun would be 

 much greater than that outside of the sun's disk. Most of the light 

 would be received from the portion of the sun at a very small angular 

 distance. A point between us and the sun would be illuminated in all 

 directions, that is. through the entire 3G0°. A point outside the sun's 

 disk could at most receive light only from 180°. Moreover, the edge 

 of the sun is much fainter than its centre, which would still farther 

 reduce the light. "We should then expect that the light received from 

 Venus would be greater than that of the sky near the sun's limb, the 

 opposite result from that indicated by the observations. This effect 

 would be modified by the solar atmosphere, which would increase the 

 light outside of the sun. The observations of Professor Langley, 

 however, during the eclipse of 1878, seem to prove that the light of 

 the corona is entirely insufficient to produce this effect. The difficulty 

 of photographing the corona confirms this view, but its spectrum in- 



