8 ANNIVERSARY ADDRESS. 
Mr. Christie, using the large spectroscope of the Greenwich 
Observatory, has confirmed Dr. Huggins’s marvellous discovery of 
the proper motion of stars, but he has not been able, any more 
than Dr. Huggins, to find proofs of motion in the nebule. No 
doubt this is owing to the inherent difficulty of getting exact 
measures of the bright lines of faint spectra. Dr. Huggins has 
this year made another advance in the examination of star spectra, 
and has succeeded in so improving his apparatus that a star can 
be kept on the slit of the spectroscope until a photograph is 
taken ; and he has secured the finest photographs of star spectra 
yet obtained. The advance thus made is most important, for the 
spectra can now be measured and compared at leisure, free from 
all the difficulties which beset the direct analysis of the star- 
light. 
In the magnificent physical observatory which has just been 
constructed at Potsdam, near Berlin, no expense has been spared 
to provide it with the best optical instruments. Dr. Vogel, the 
director, says that the spectroscope made by Hugo Schroder, of 
Hamburg, is a splendid instrument, and its twenty-one single 
prisms, combined into a system on Rutherford’s plan, will enable 
him to measure one-hundredth part of the space between the D 
lines, and shows in the same space nine fine lines. When I saw 
this statement recently published, it recalled the information I 
had given you in November, 1875, viz., that nine fine lines had 
been seen at Berlin between D 1 and D 2 with the spectroscope 
then in use; that some years previously (1868) Dr. Huggins, 
using the great spectroscope at Oxford, saw twelve lines in the 
same space; and that Colonel Campbell, with a spectroscope 
made by Hilger, had in London seen nineteen lines between the 
4wo Ds ; and comparing these statements with my own experience 
here, using a much finer spectroscope (also by Hilger, of London) 
which shows me seven lines. between the two Ds, I was led to 
think that these differences must be due to atmosphere, and not 
to the quality of the spectroscope. If so, it is a most important 
question to determine; and I have therefore carefully examined 
these lines with our large spectroscope, which has a dispersion 
4 
