524 Scientific Proceedings, Royal Dublin Society. 
About the sixties, the invention of Foucalt of using silvered 
glass mirrors, instead of metallic, gave the reflector a new lease of 
life, until it was found that the silver film was very perishable in 
the neighbourhood of towns. Moreover, the advance made in 
the manufacture of optical glass, has rendered it possible to 
obtain discs of a size almost comparable with the large reflecting 
telescopes. 
It was about this time that the author published the Paper 
referred to, thinking that the deserved popularity of the refractor 
had caused the many strong points of the reflector to be overlooked. 
The strongest advocates of the refractor have, undoubtedly, 
been in the New World, where most of the great modern refrac- 
tors have been built and installed, and it is therefore somewhat 
surprising, but at the same time gratifying to find from the Paper 
referred to in the title of this Paper, that American astronomers 
are becoming quite alive to the many strong points of the re- 
flector, and that a Paper from the pen of one so eminent in his 
profession as Professor Hale should contain such a remarkable 
confirmation of the views expressed in the author’s Paper referred 
to above, and published twenty years ago. 
Professor Hale, in his Paper, treats the subject in a far more 
exhaustive and complete manner than that which the author was 
able to do twenty years ago. He points out in the first place the 
actual necessity that exists for the building of telescopes larger 
than we have as yet attempted, if we desire to solve some of the 
great astronomical problems of the day ; he says :— 
“‘ But those who wish to materially reduce the probable error 
of wave length determinations of iines in stellar spectra, either for 
the purpose of increasing the accuracy of line of sight measure- 
ments or to render possible such detailed studies of certain lines as 
are now made in the solar spectrum, must be content to wait for 
the construction of telescopes much larger than the great instru- 
ments of the present day.” 
He then points out the particular classes of observations which 
the reflector is suitable for, and shows that that perfection of 
definition which the imperfection of our atmosphere renders 
impossible is not necessary in these particular observations. 
