8 Mr. Howsrp Grusz on Great Telescopes of the Future. 
Advantages of Reflector over Refractor. How influenced by increase of size. 
B 1.—This advantage increases rapidly with increase of size, so much so that 
Dr. Huggins, when testing the 15-inch objective, thought at first it was over- 
corrected for achromatism, and Professor Newcomb, I believe, had the same experi- 
ence when the great 26-inch objective was first tried. And this outstanding colour 
in the image will in large telescopes create a great inconvenience in all spectro- 
scopic observations, as the star requires refocusing on the slit for every different 
ray of the spectrum. 
B 2,.—This point cannot be said to be influenced by increase of size, but the 
value of the advantage of the Reflector becomes undoubtedly much greater by such 
increase, when we consider the number of unresolved problems in Physical 
Astronomy, and that these are resolvable only by increase of optical power. 
B 3.—This is a point which only comes into effect in large sizes. 
A Reflector can be, as mentioned before, perfectly supported irrespective of size. 
Refractors can only be supported round their edge. 
Up to 12 inches diameter I have found it possible to support objectives without 
flexure on three points round the periphery. 
When, however, I tried 15 inches, I found decided flexure from the three points, 
and was obliged to introduce three intermediate levers of support, automatic in 
their action. With the 27-inch Vienna Refractor I expect I shall have to use six 
intermediate supports. 
A question now arises, what is the greatest size which can be supported by edge 
support alone? For a point must sometime be arrived at, at which the weight of 
the glass will be sufficient either to distort the figure sufficiently to injure the 
definition, or to produce such a molecular strain as to polarize the. light, and 
produce a double image. 
Then arises another question : suppose this point arrived at, is there no remedy ? 
Two such occur to me— 
Firstly--A central support could be introduced with no worse effects than the 
central mirror in a Reflector ; and 
Secondly—A far more elegant contrivance would be to hermetically seal the tube 
(the lower end might be sealed by an equivalent to a low power Barlow lens), 
and fill the tube with air under such pressure as would support a sufficient portion of 
the weight of the objective on a perfect air cushion. Of course this pressure should 
be regulated according to the altitude of the telescope, but I have devised a 
mechanical contrivance for this purpose. 
The pressure required would be very small. Suppose the objective to be forty 
inches aperture, and 600 Ibs. weight, and that it was proposed to lift two- 
thirds of its weight on the air cushion, a pressure of about one-third of a pound on 
the square inch, or say one-fiftieth of an atmosphere, would suffice, even when the 
telescope is at its maximum elevation. 
