808 The Past and Coming Transits and Arctic Explorations. 



circumference was completed by meaug of a vivid, but narrow and 

 ill-defined, border of light, which illuminated that part of her circum- 

 ference which was off the sun. He adds, that it disappeared two or 

 three minutes before the internal contact. A similar phenomenon was 

 witnessed during the same transit by Wales and Dymoud, at Hudson's 

 Bay, by Pingr6 and De Fleurien, at Cape Francis, in the Island of St. 

 Domingo, and by various other observers at different places. 



A little consideration will suggest the true cause of this appearance, 

 and show that its effect on the observation of internal contact can not 

 but seriously affect the accuracy of the timing. No light can show 

 itself round the portion of Venus outside the sun, between the 

 moments of exterior and interior contact, unless the planet has an 

 atmosphere capable of refracting the solar rays ; but, if the planet 

 has such an atmosphere, the observed effect can not but be produced. 

 If we suppose an observer on Venus at a point F, on the part of her 

 limb most remote from the sun — P not being the point which is seen 

 from the earth at that part of the limb, but so placed that the true 

 horizon-plane for an inhabitant of Venus there is parallel to the line 

 from the observer on earth to the center of Venus — then, if there 

 were no atmosphere, an observer at P could see neither the sun nor 

 the earth, at least, not where the terrestrial observer is placed. The 

 sun would be just below the true horizon of the observer on Venus, 

 and so would the observer on earth ; and d foHiori the observer 

 on earth would not be able to see the sun round the part P of Venus. 

 This part would be, as it were, the summit of a hill, from which the 

 sun on one side and the earth on the other would be invisible, and 

 therefore invisible from each other. But, if there is an atmosphere on 

 Venus resembling our own atmosphere in its effects, the observer at 

 P would see the sun raised by refraction above his horizon, and the 

 earth, directly opposite, raised considerably above the horizon — pre- 

 cisely as at the time of total lunar eclipse — the observer on earth, if 

 so placed that the eclipsed moon is apparently just above the horizon 

 (really raised by refraction), can see the sun also directly opposite the 

 moon, and raised wholly above the horizon. And as the line of sight 

 from the observer at P to the sun on one side, and to the earth on the 

 other, is thus twice bent by refraction, so the line of sight from the 

 observer on earth to P is curved doubly as it passes P, and the sun is 

 brought into that observer's range of view. The same is true for all 

 points round that part of Venus' limb outside the sun, the atmosphere 

 of Venus bringing greater and greater quantities of sunlight round 

 the dark limb the nearer the part of the limb is to the sun. Thus, there 

 is seen round the arc of Venus, outside the sun, a ring which is not 



