THE STORY OF TRANSITS 435 



But this is not quite so simple a process as it seems, for the 

 planet is not a point and does not appear upon the solar disc 

 suddenly. When it just touches the outside edge we have 

 what we call the " external contacts," whilst when the planet is 

 just completely on the sun we have " internal contacts." There 

 is a very great difficulty in observing these contacts exactly, 

 and the actual instant of contact is doubtful by about ten or 

 fifteen seconds, a very big error in work of this kind. The dif- 

 ficulty is, in part, due to optical defects both in the eye and in 

 the telescope, for when two bodies approach very close to each 

 other their surfaces seem to run together. 



A simple experiment illustrates the idea. Place the thumb 

 in contact with a finger and hold them two or three inches 

 from the eye. If they now be separated slowly, it will be seen 

 that they tend to cling together and a kind of ligament, or 

 " black drop," is observed between them. 



We have precisely the same kind of effect when Venus is 

 entering or leaving the solar disc, so that it is quite impossible 

 to see the actual instant of contact, although previous practice 

 with an artificial " Venus " enables some kind of allowance to 

 be made for it. 



A very similar difficulty is produced by the planet's atmo- 

 sphere, which appears as a luminous ring round Venus. There 

 seems to be no way of getting over this trouble, which itself 

 makes the moment of contact uncertain to the extent of five or 

 six seconds. 



There are two great objections to this method proposed by 

 Halley. The one is that the observers must be near the poles 

 of the earth in places that are necessarily difficult to reach and 

 unpleasant for work. The other is that both the beginning 

 and the end of the transit must be seen, so that the weather 

 must be favourable for several consecutive hours at the right 

 time — a very big demand in a polar region. 



Both these objections are avoided in the method proposed 

 by Delisle. In the first place, the two observing stations are 

 chosen near the Equator on a line more or less parallel to that 

 in which Venus is moving ; in the second place, one contact 

 only need be seen, and this may be either the beginning or the 

 end of the transit. It must also be remembered that the weather 

 is much more likely to be fine near the Equator than it is near 

 the poles. 



The principle of the method is simple. Let A and B be 

 two stations on the Equator, so placed that the line joining 

 them is a diameter of the earth. When Venus is in some posi- 

 tion V, the observer at A will see it make its first contact ; he 

 notes the exact time, though, of course, he is faced with the 

 " black-drop " difficulty. The observer at B also notes the 



