PREPARATIONS EOR THE TRANSIT OF VENUS. 
243 
the photographic method, applied at stations where the whole 
transit can be seen . The condition italicised is of some 
importance as indicating a distinction between two possible 
photographic methods, one corresponding to Delisle’s, the other 
to Halley’s. The English arrangements, for example, include 
the application of photography at the Sandwich Islands, where 
only the beginning of the transit can be seen, and at Alex- 
andria, where only the end can be seen. Such photographs 
can, of course, only have value when the absolute time at which 
each is taken is accurately known. On the other hand, the 
American stations are so selected that photographs can be 
taken throughout the whole continuance of the transit, and 
these, by indicating the chords of transit, will have a value 
independent of the exact determination of absolute time ; so 
that, in fact, the original error of the Astronomer-Royal (that 
Halley’s method fails totally in 1874) would, if not corrected, 
have affected the application of the photographic as well as 
other methods. Fortunately, the American astronomers have 
not been misled by it,* and at all the stations they propose to 
occupy, the whole transit will be visible, as will presently be 
more particularly noted. 
The plan they adopt for photographing the sun differs 
essentially from that which European astronomers propose 
to employ. “For the purpose of obtaining an enlarged image 
on the photographic plate,” writes Professor Hilgard, of Wash- 
ington (describing the ordinary method), “the image of the 
sun, after being formed in the focus of the telescope, is enlarged 
* At the same time I must remark, that it seems to me altogether proper 
that the Delislean stations in question should be occupied by photographers. 
What I have all along insisted upon has been, the necessity of employing 
every available method, and occupying every available station ; and it 
would have been a matter to be regretted, had any one of the regions origi- 
nally suggested by Sir G. Airy been neglected. I note this because I heard, 
during my stay in America, some rather too severe comments on the Astro- 
nomer-Royal’s urgently-expressed desire that the Americans should establish 
two observing parties in the Delislean region, of which the Sandwich Islands 
give the leading stations. It was doubtless a mistake on Airy’s part to de- 
sire that this region, already amply provided for, should be occupied instead 
of Halleyan stations altogether more advantageous ; and it was well that 
the American astronomers declined to change their plans. Yet the occupa- 
tion of this Delislean region, as well as the other Halleyan stations, was 
equally proper. An eminent American humorously compared Sir G. Airy’s 
request to the well-known appeal of the fox who had lost his tail: “Your 
Astronomer-Royal,” said he, “ who has lost his head over Delisle’s method, 
seems to wish Newcomb to lose his also.” But I must confess I should 
have been very sorry to have seen the Delislean stations insufficiently pro- 
vided for. 
