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by a lens or camera to the desired size, the photoheliographs, 
as they are called, being thus enlarged to a diameter of 
about four inches. This plan has been adopted for the 
photographic apparatus to be used by the British, German, 
and Russian parties commissioned to observe the transit of 
Venus. A different plan has, however, been adopted for the 
American parties, with the view of avoiding some difficulties to 
which the former method may be thought subject. These are 
conceived to reside in the fact that not only all imperfections 
in the focal image are thus enlarged, but that the optical 
imperfections of the camera are superadded. To avoid this 
objection it was deemed best to make the telescope so long 
that the image formed in its principal focus would need no 
further enlargement. Here another difficulty presented itself. 
The telescope must be forty feet in length in order to give an 
image four inches in diameter. Such a telescope, pointed at 
the sun, would scarcely be manageable. Hence the plan was 
devised, which Professor Winlock was the first to put into 
practical operation. It consists in fixing the long telescope in 
a horizontal position, and reflecting the sun's rays into the 
object-glass by means of a plane glass mirror, moved by 
clockwork, so as to throw the image of the sun continually 
into the telescope. This need not be done with great preci- 
sion, since, as has already been said, the time of exposure is 
exceedingly small, and the mirror can at any time be adjusted. 
It is obvious that, in this arrangement, as much depends upon 
the perfect figure of the mirror, as in the other upon that of 
the enlarging lens ; but it is, doubtless, an advantage that 
different methods should be employed, so long as a sufficient 
number of stations are occupied to give an independent result 
for the sun’s distance from observations by each method alone, 
since such only can be considered as strictly comparable. This 
condition is amply fulfilled by the abundant provision made by 
the American Government for the observation of the important 
-event in prospect.” 
I may remark, however, that Professor Newcomb, with whom 
I had the pleasure of a conversation relative to the subject, 
attaches very great importance to the advantages of the 
.American method. He remarked that by employing this 
method the astronomer is enabled to measure the distance of 
Venus from the sun’s centre with an exact knowledge of the 
value of the deduced distance, because, the focal length of the 
telescope being known, the value of any distance indicated in 
the focal image is at once determined. All that is necessary, 
then, is to determine the centre of the solar image, which can 
be safely done by measurements made from the limb. Manifestly 
no photographic effects affecting the position of the limb in 
