PREPARATIONS FOR THE TRANSIT OF VENUS. 
245 
the photograph could appreciably affect the determination of 
the centre, even though such effects were not absolutely 
uniform all round. But in the ordinary method of photo- 
graphing the determination of the arc-distance of Venus from 
the centre is not reliable (in a problem of such extreme 
delicacy), because the estimated dimensions of the solar image 
could not be accurately determined, while the observed 
dimensions, being determined from the photographic limb of 
the sun, would be affected more or less by photographic 
irradiation. No apparent sharpness of the limb can render 
certain the fact that the limb in the photographic image 
corresponds to the true solar limb. I must confess that Pro- 
fessor Newcomb’s reasoning ^eems to me irrefutable. It will 
be observed that it does not depend on practical or technical 
knowledge of photography, since the photographic irradiation 
demonstrably exists, and is demonstrably variable in amount. 
In a conversation with Dr. H. Draper, of New York, whose 
experience in those matters is well known to be unsurpassed, I 
found Professor Newcomb’s doubts fully confirmed. It is true 
that Dr. Rutherfurd, whose great practical experience in solar- 
photography is unquestioned, agrees with his eminent British^ 
rival in such work, Dr. De la Rue. But then it is to be 
remembered that both Rutherfurd and De la Rue view the 
matter as photographers, while Newcomb and Draper view it 
from an astronomical standpoint, and in this case the astrono- 
mical, not the photographic relations, are chiefly in question. 
We do not want handsome solar pictures, but pictures which 
can be confidently measured; and certainly the plan adopted 
by American astronomers is that which best meets this 
requirement. I may add that a very eminent American astro- 
nomer, speaking to me on this subject, made this strong remark,., 
“I regard the photographic method adopted by the British, 
astronomers as involving a mistake as fatal as Airy’s original! 
mistake would have been if uncorrected. ” *. 
The programme of the American expeditions is as follows : — - 
Eight parties will take the field, three in the northern 
hemisphere, 66 where the meteorological conditions are supposed 
to be somewhat more favourable than at corresponding stations 
in the southern hemisphere,” where there will be five. The 
three northern stations will be (1) at Vladivostok, in Siberia; 
(2) at Tien-tsin, in China ; and (3) at some as yet undeter- 
mined place in Japan. Originally, in response to the strongly 
expressed wish of the Astronomer-Royal that two Delislean 
* Lord Lindsay, it is to be noted, will employ the same method as the 
American astronomers, after carefully testing, in a series of photographic 
experiments, the reliability of the two methods. 
