SCIENTIFIC TRANSACTIONS 
OF 
THE ROYAL DUBLIN SOCIETY. 
VOL, I. (NEW SERIES). MEMOIR No. I. 
ON GREAT TELESCOPES OF THE FUTURE. 
BY 
HOWARD GRUBB, F.r.a.s., Honorary Master in Engineering, University of Dublin. 
I propose in the present communication to consider the relative advantages and 
disadvantages of the Refracting and Reflecting Telescope, in what manner those 
advantages and disadvantages are likely to be modified by any great advance in 
size over those at present in existence, the practical difficulties of construction in 
each case, and the most promising means of overcoming those difficulties. 
Let us first consider the advantages which Refracting Telescopes, of such sizes as 
are already in existence, possess over Reflecting Telescopes. 
Ist. Inasmuch as the light lost by the double reflection in case of a Reflector vs 
greater than the amount lost by transmission through an object glass of 
corresponding size, a Reflector to be of same optical power must be of 
greater diameter than a Refractor; consequently, rt will be subject to 
greater atmospheric disturbance and will not be useable with as low a 
power as the Refractor. 
This is a most important point and worthy of attentive consideration ; for, inas- 
much as atmospheric disturbance is really the great limit to optical power—and 
a limit also which opticians and astronomers are powerless to modify—it seems 
evident that whatever direction enables us to advance our optical power with the 
least increase of atmospheric disturbance must be the most desirable. By-and-by, 
however, in considering the effect of increase of aperture, we shall see that this 
advantage of Refractors over Reflectors must diminish as the telescopes get large, 
and at some point not very far remote, must vanish altogether. 
Let us consider this point a little further. Dr. Robinson, in “ Philosophical 
Transactions,” 1869, vol. 159, treats of this matter, and the result of his calcula- 
tions may be given in a few words. 
The light-grasping power of a Reflector may be expressed by the square of the 
aperture x a certain fraction which represents the proportion of the light reflected 
to that of the whole light. 
B 
