PREPARATIONS FOR TIIE TRANSIT OF VENUS. 
241 
tion, either at the beginning or end, would in most cases be 
useful for the method next to be considered, called Delisle’s, 
because a good Halleyan station is commonly a fair Delislean 
station, and may be (in fact, very commonly isj a very good 
station for Delisle’s method. 
Delisle devised his method to meet the two objections men- 
tioned at the beginning of the preceding paragraph, which 
chanced to apply with particular force to the transit of 1761. 
In fact, he found, when studying the conditions of that transit, 
that Halley’s method would fail totally for it. He suggested 
that instead of the duration of the transit the absolute time at 
which the transit began or ended should be determined. It is 
clear that a northern observer so placed that Venus would, as 
seen by him, travel along a certain chord, or a portion of such, 
would see the transit begin at a different time than would a 
southern observer, from whose station she would appear to 
traverse a different chord, either longer or shorter, according as 
the transit took place across the lower or upper portion of the 
sun’s disc. The difference of time so caused would be less or 
greater according as the chords were nearer or farther apart. 
So that the determination of this time-difference involves the 
determination of the distance between the chords in this, pre- 
cisely as in Halley’s method. And a similar remark applies to 
the observation of egress by two observers, one at a suitable 
northern, and the other at a suitable southern station.* 
The chief difficulty presented in Delisle’s method consists in 
the fact that it requires the longitude of each station to be 
most accurately determined, and also the exact moment of local 
time when the transit begins or ends, as the case may be. This 
difficulty is analogous to one of those already considered in 
dealing with the direct method. 
Lastly, there remains a method which modern progress has 
rendered available, viz., the photographic method. This 
method, viewed astronomically, requires no explanation, since 
it amounts merely to the substitution of photography for 
observation in applying the “ direct method.” It removes 
the second difficulty considered in dealing with that method, 
leaving the first untouched, though the possibility of taking 
many photographs during the progress of the transit, and of 
thus determining the chord of transit, is a manifest advantage 
possessed by the photographic method. 
* Here, as in considering Halley’s method, I have overlooked for the 
moment the dimensions of the disc of Venus. Of course the observer 
cannot note the passage of Venus’s centre over the edge of the sun’s disc, 
but must observe the contact of her disc and the sun’s, either on the inside 
or on the outside. 
VOL. XIII. — NO. LII. R 
