ANNIVERSARY ADDRESS. 7 
once, and that once perhaps interrupted by a passing cloud, Pro- 
fessor Harkness urges the use of this method, and thinks it would 
prove as good as the contact observations, where the acknowledged 
uncertainty amounts to 0:15". He says :—“The photographic 
method cannot be defeated by passing clouds, is not liable to any 
uncertainty of interpretation, seems to be free from systematic 
error, and is so accurate that the results of a single photo. has a 
probable error of only 0°553. If the sun is visible for so much as 
15 minutes, thirty-two photos. of the sun can be taken, and these 
will give as accurate a result as the observations of both internal 
contacts. In view of these facts, can it be doubted that the photo- 
graphic method offers as much accuracy as the contact method and 
many more chances of success?” 
The suggestions made by Professors Harkness and Pritchard 
have been strengthened by a paper read before the Royal 
Astronomical Society, by Mr. Maunder, who suggests as a method 
of avoiding the uncertainty in the measures of the photos. that 
they should be so taken as to show all the details of the sun’s sur- 
face, and then Venus could be referred to a spot or other markings ; 
‘in fact, that the distance measured on the photos. should be as 
small as possible. The idea was well received in the Society, 
though it does not seem to entirely avoid the difficulty, for the 
position of the spot must be determined, and this is almost 
impossible with the English instrument, as stated above. The 
American photoheliograph is nearly free from distortion of the 
field ; and if at this eleventh hour it should be decided to make 
use of it, it will be too late for Australia to send to Europe 
for the instruments, and we should have to be content with 
the one I have, which ison the American plan, but has a longer 
focus, and would therefore give a larger and better picture. 
_ With reference to the probable value of the solar parallax, 
I take the following from an important paper published in 
November, 1881, by Professor Harkness, in which he discusses 
the relative censmer ok ali: the ti Sores methods ehdeaesinine 
