102 TEAXSIT OF VENUS. 



sun, which for some seconds kept me in a state of uncertainty as 

 to the true time of actual ingress, the shading did not break 

 abruptly, but seemed to melt away in such a manner as to leave 

 a doubt in my mind of the exact time the planet passed the edge 

 of the sun." 



Mr. Savage says : — " The definition at this point being so bad 

 between the limbs of the sun and planet, it is quite likely the 

 time given by me might be about ten seconds late ; the edges at 

 contact then became very dark. As the planet advanced on the 

 sun's disc a little way, this shading still connected the planet and 

 the sun's edge ; but that portion of it nearest to the planet 

 showed indications of Hiding away gradually until at length it 

 disappeared altogether without any sudden break." 



It is evident that what we have here described is a phenomenon 

 very different from that which is known as the hlacTc drop, for 

 here the uncertainty lasts much longer, and does not occur when 

 the limbs are apparently separated, but when they are in fact as 

 well as appearance in contact and slightly overlapping ; and while 

 this phenomenon is clearly made out to have lasted about four 

 minutes, Mr. Stone, Astronomer-Eoyal at the Cape of Good 

 Hope, and the best authority on this subject, estimated that the 

 black drop would only last eighteen seconds. 



I cannot see any possible explanation of this appearance 

 except that which is aftbrded by assuming a considerable atmo- 

 sphere around Yenus, and there are some other facts noted on 

 evidence too strong to overlook, which certainly seem opposed to 

 this supposition. 



If, however, for the present we assume that Venus has an 

 atmosphere something like that of the Earth — say, of 40 miles 

 extent — it would, on the day of transit, have subtended an angle 

 of only one-third of a second of arc, or much too small a quantity 

 to be recognized, unless as a bright line on a dark back-ground, 

 or by its effects in diflusing light, and it is in this way that it 

 helps us with an explanation of this hazy phenomenon. Such a 

 faint line of atmosphere, so long as it crossed the cusps at a con- 

 siderable angle, would have no appreciable effect on the cusps, 

 and they would remain as they wei'e seen, quite sharp ; but when 

 the limbs of the sun and planet were near the point of contact 

 the cusps would become very much attenuated and for a con- 

 siderable extent quite within the power of our assumed atmo- 

 sphere, and the efiect would be manifest, not so much from the 

 diameter of the light cut off, as from the length of the cusps so 

 affected ; and it is remarkable, as bearing upon this view, that 

 the cusps were apparently seen through the haziness ; and on 

 myself and other observers the impression produced was similar 

 to a want of focal adjustment ; and the gradual clearing up, 

 melting away, &c., which we have heard described is exactly what 



