556 PHILOSOPHICAL TRANSACTIONS. [anNO I76I. 



At 8*^ 16"^ 4V No diminution of light between the limbs of Venus and the 

 sun. 

 8 16 42 Slight penumbra, or diminution of light, near where the con- 

 tact was to be. 



43 Penumbra of a grey colour, near the same place. 



44 Penumbra almost brown, and the thread of light very narrow. 



45 Penumbra brown, and the thread of light in the contact point 

 indistinct, or lost. 



46 Penumbra more brown, and the touch the smallest possible. - 



47 Penumbra almost black, and the touch a little broader. 



48 Slight black in the point of contact, and the edges a little 



broader. 

 8 16 49 True black in the point of contact, and the edges a little 

 broader. 



Here he guessed that observers would differ in their judgment about the mo- 

 ment of contact some seconds of time, or that some would estimate the contact 

 sooner than others. 



From these observations he concluded that the thread of light in the point of 

 contact was so obscured as to be undiscemible at 8^ iG'" 46*, and that true 

 black did not succeed in the same point till 3* after, namely, 8^ 16"" 49*; and 

 from* both of these properties he concluded that the real internal contact was at 

 gh |gm 47s by the clock; which makes 8^^ 16'" 11* equal time, and 8^ 18™ 2* 

 apparent time at Chelsea ; and 8^ 18"" 43* apparent time at Greenwich. While 

 Venus was on the sun's limb no other penumbra appeared between the limb of 

 Venus and the sun, than had appeared before on the sun's disk ; and therefore he 

 concluded there must be an atmosphere about Venus, which receiving weak im- 

 pressions of light between the limbs of Venus and the sun, occasioned the un- 

 certainty of ascertaining the exact instant of the internal contact as above 

 described. 



* As the 6-feet Newtonian telescope magnified 4 times as much as that of the 2-feet Gregorian 

 telescope, and the vanishing of the thread of light, from its least degree of duskishness to a true 

 black, was about 3 seconds of time by the 6-feet telescope, the time in which the thread of light 

 was vanishing from the least degree of duskishness to a true black, by a 2-feet Gregorian reflector, 

 may be supposed to have been 4 times 3=12 seconds of time ; and hence an error, or rather dif- 

 ference of pronunciation, but not of judgment, may have arisen among good observers, if some 

 estimated the contact by the invisibility of the thread of light, and others by an apparent blackness 

 in the point of contact j or, which is the same thing, the time when the planet had made the least 

 apparent dent in the sun's limb, of the same colour, through a dark glass, as the sky. This was 

 verified by a 2-feet Gregorian reflector, in the contact above mentioned, and possibly may have od- 

 casioned greater difi'erences in estimating the contact, with lesser telescopes, to no less than half a 

 minute of time. — Orig. 



