2 Mr. Howarp Gruss on Great Telescopes of the Future. 
This varies in various forms. In the Cassegrain, curious to say, the fraction is 
higher than in the Newtonian, and in the silver on glass mirrors considerably 
_ higher than in the metallic.* 
In the Refractor the light-grasping power may be expressed by the square of 
the aperture x a fraction which represents the balance of light passed after absorp- 
tion from the substance of each glass and reflection from the surfaces of each glass. 
It is evident from the foregoing that the light-grasping power of a unit of surface 
of a Reflector is independent of the size, while in a Refractor it diminishes as the 
size increases, on account of the extra absorption of light, from the extra thickness 
of glass. Dr. Robinson estimates, from experiments made with the best existing 
kinds of glass, that a Refractor of 35°435 inches would be just = a Reflector of 
same size (metallic), but that beyond this size the Reflector would have the 
advantage. 
The next point of advantage of Refractors we have to consider is that of— 
2nd. The greater permanence of collimation and consequent sutability for ordinary 
observatory work and measuring purposes. 
This is, no doubt, a considerable advantage of the Refractor, particularly if it is 
intended that the instrument should supply the place and do the work generally 
allotted to the great Equatorial Reflectors placed in our large observatories. It 
is, however, most probable that any telescopes constructed in future of larger 
dimensions than those at present in existence will have such special work allotted 
to them as will render them quite independent in this matter. This defect (if it 
may be called such) arises from the impossibility of supporting large Reflectors 
rigidly in their cells, either as regards their back or lateral supports. The defect 
becomes, of course, more apparent in large size telescopes, but can probably be 
much reduced by slight modifications of the supporting systems. For instance, I 
have found great advantage in grinding the back of the mirror to a convex curve 
with radius = that of the face + thickness of dise. Any error in collimation is 
thereby much reduced. 
It is quite possible and even probable that if the size of Refractors be much 
increased, the errors of collimation may become, even in their case, of serious 
quantity. As to comparing the relative qualities ‘of Reflectors and Refractors as 
regards suitability for ordinary measuring purposes, it seems to me quite unne- 
cessary, as this work is capable of being done, and done efficiently, by smaller 
instruments, and any measurements likely to be useful for such monster telescopes 
would only be those differential measures taken by means of micrometers, which 
errors of collimation, &¢., do not affect. 
*T refrain from giving any actual figures here because there is much difference of opinion thereon, 
and I hoped that before this paper was read that the Photometrical Committee of the British Association 
would have come to some reliable conclusion on the subject. 
