186 The Late Transit of Venus. [April, 
of the sun and Venus, obtained by De la Rue’s method, as 
is requisite in the delicate problem of measurement involved. 
The photographic delineation ought to be not only the best 
possible, but ought to be practically perfect. What, how- 
ever, do we hear (when it is too late) from the very person 
who was responsible for the employment of the best available 
method, and at whose instance not only the English, but the 
Russian and German, arrangements for photographing the 
transit had been selected? The Astronomical Society is 
calmly informed that the French method of photographing 
the transit (on silver) is ¢mfinitely more exact than the methed 
of photographing on glass. I do not know in what sense 
the Astronomer Royal used the word infinitely; but he is a 
mathematician, and therefore probably used the word in its 
mathematical sense; that is to say, regarding the trust- 
worthiness of the French photographs on silver as repre- 
sented by some finite quantity ¢, the trustworthiness of the 
photographs taken by the English, Russian, and German 
Government parties, is in the Astronomer Royal’s opinion,— 
t 
SS) = '0) 
[o-) 
We cannot blame Mr. De la Rue for proposing a method 
thus rejected as utterly untrustworthy. He was not the 
responsible person. Dr. Rutherford, in America, advocated 
the same method, which probably seems to the photographer 
the best available. But the American astronomers respon- 
sible for the photographic arrangements rejected this method, 
showing by their calculations that it could not be trusted. 
Our official astronomers have also found this out. But 
there is this trifling difference—American official astronomers 
made the discovery before selecting a method for photo- 
graphing the transit ; ours made the discovery a month after 
the transit had taken place. 
The Americans met the difficulty, not as the French did, 
by taking daguerreotypes, but by using an object-glass of 
great focal length (40 feet). It is manifest that with such 
an arrangement, the true focal length being known, the 
scale of the sun picture is determined independently of the 
apparent size of the solar image. For the focal image is as 
large as a disc which, seen at the focal distance, would look 
just as large as the sun; and the sun’s apparent diameter at 
the time of transit being known, as also the true focal 
length, we can calculate how large such a disc would be. 
The photographic picture may be slightly larger or slightly 
smaller through photographic defeéts,—expansion by irradi- 
ation or shrinkage all round the border; but, as we have no 
