PREPARATIONS FOR THE TRANSIT OF VENUS. 
239 
transit of this year the element of time was concerned in such 
a way that it was impossible to wait beyond a certain point, for 
the controversy related to the choice of methods for observing 
an event which could not be postponed. I had, therefore, good 
reason, after waiting from 1869, when the matter was first 
mooted, to the spring of 1873, some twenty months before the 
transit was to take place, for urging the views which seemed to 
me correct, and which have since been admitted by all to be so, 
as earnestly and as publicly as possible. But so soon as the 
time came when the final changes had been made, and the plan 
of operations definitively decided upon, the discussion of the 
original scheme or of the modified one, and the consideration 
of disadvantages which would have resulted from one, or of 
advantages which might have been obtained from yet more 
extensive modifications, was henceforth improper. Certainly 
they would be improper on my part ; nor do I think that 
the history of this matter requires that any stress should be 
laid by others on mistakes which have been in the main cor- 
rected, or on the threatened loss of opportunities which will 
not now be allowed to pass away unutilised. 
For fortunately a far pleasanter task than any on which I have 
hitherto been engaged in connection with the transit of 1874 
lies now before me. I propose to describe how the various 
nations which take part in scientific research propose to take 
advantage of the opportunities presented by the approaching 
transit ; and the task is a pleasant one, because there is scarcely 
a single point of vantage in the whole field of operations which 
will not be adequately occupied. 
Let us in the first place consider the methods available for 
observing the approaching transit, then the regions where these 
various methods can be most suitably applied, and lastly the 
dispositions made to occupy in suitable force those favoured 
regions. 
It is hardly necessary to enter - here at any length into the 
principles on which the determination of the sun’s distance by 
means of the observation of Venus in transit, depends ; because 
the whole subject has been already very fully dealt with else- 
where, and is sufficiently simple. The following mere outline 
of the matter may, however, be permitted. If two observers, 
separated by a sufficient distance on the earth, see Venus in 
transit at the same moment, they will see her disc projected on 
the different parts of the sun’s face ; and it is manifest that 
if the distance between the two observers on the earth be 
known, the displacement of Venus on the sun’s face, if observed 
with sufficient accuracy, will give the means of defining the 
distance of Venus, and thence (by Kepler’s third law) the dis- 
tance of the sun. 
