1875.] The Late Transit of Venus. 187 
occasion to measure the diameter of the disc, this is a 
matter of perfect indifference. All we require to know is 
the distance, in any picture, between the centres of the 
discs of the sun and Venus, and whether the outlines of 
these discs have contra¢ted or expanded the positions of 
their centres can be accurately determined. The great 
difficulty to be surmounted consisted in the construétion of 
suitable mirrors for the heliostats. An ordinary telescope, 
40 feet in length, was out of the question, since no available 
machinery could direct such a telescope continually towards 
the sun. The construction of a flat mirror was a matter of 
extreme difficulty. As Prof. Newcomb pointed out—‘ The 
slightest deviation from perfect flatness would be fatal: for 
instance, if a straight-edge laid upon the glass should touch 
at the edges, but be the 100,o00th of an inch above it at the 
centre, the reflector would be useless.” But with such 
opticians as Alvan Clark and his sons this difficulty was not 
found insuperable, and the mirrors provided for the eight 
American parties were to all intents and purposes perfect. 
Lord Lindsay selected the same method. The mirror for 
his heliostat was constructed by Repsold, and worked excel- 
lently. 
We must, then, in considering the photographic results, 
attach chief—if not sole—value to the successes obtained 
by the Americans and Lord Lindsay using the long focal 
method, and by the French using Daguerre’s process. For- 
tunately a sufficient number of results have been obtained 
by both methods, and in both hemispheres, to ensure the 
determination of the solar parallax to a much greater degree 
of accuracy than heretofore. I think it is not too much to 
hope that the sun’s distance may now be ascertained within 
limits of error not exceeding 100,000 miles on either side of 
the true distance. 
On the whole, Science has every reason to congratulate 
herself on the results achieved during the observation of the 
late transit. There was a good deal of blundering at the outset, 
and there were some points to be regretted in the final arrange- 
ments, but the more important mistakes were corrected in 
good time. In portioning to the different nations the 
honours due to them, we must assign to some countries 
special credit for some matters, while in other matters other 
countries took the lead. America devoted a larger sum to 
the expenses than any two nations together, and adopted 
excellent arrangements. Russia provided for the greatest 
number of stations; in fact, for more than England, 
America, and France together. Germany alone combined 
