Mr. Howarp Gruss on Great Telescopes of the Future. 7 
B 3.—Possibility of supporting Reflectors with perfect freedom from flexure, 
arrespective of size. 
No matter what the size of the mirror may be there is neither theoretically nor 
practically any difficulty in supporting it perfectly free from flexure by using my 
father’s system of levers. 
It is not so with objectives. J¢ is possible to support them go during polishing, 
but when in their tube they can only be supported round the edge, and it is possible 
to conceive a size of objective which could be polished and figured on the machine 
to perfection, but which could never be made to perform perfectly for want of 
uniform support when in its tube. Of course, this point is not necessary to be 
considered except in case of extension of size of telescopes much beyond that 
already existing. 
B 4.—General convenience of Instrument for observing purposes. 
The tube of the Reflector is much shorter than the corresponding Refractor, 
and if the Cassegrainian form be used, the circle swept by the eyepiece is very 
short indeed, so much so that in the case of the great Melbourne Telescope of 
four feet aperture the observer never required to be more than two or three feet 
off the ground, while in the case of a corresponding Refractor the observing chair 
or platform would assume ponderous dimensions, and the observer would sometimes 
find himself thirty feet off the ground. All these matters, although spoken slightly 
of by some as “mere mechanical difficulties, easy to be overcome,” become very 
great inconveniences in practice. 
We have now to consider how these various advantages and disadvantages 
are likely to be influenced by a large increment in the size of telescopes over 
those at present in existence. 
Advantages of Refractor over Reflector. How influenced by increase of size. 
Referring back to the same numbers I used before— 
The Ist point of advantage of Refractors over Reflectors would appear to vanish 
when the objective attains a size exceeding 36 inches, and for objectives (if they be 
ever made) much beyond this the advantage would lie the other way. 
_ No. 2.—This advantage of Refractors will probably increase in quantity with 
increase of size, but, for reasons stated before, the value of the advantage will 
probably be found to diminish. 
No. 3.—This advantage undoubtedly increases with increase of size. 
No. 4.—Need not be considered for reasons before stated. 
No. 5.—Would not be affected by increase of size. 
No. 6.—May probably, for reasons before stated, vanish in very large instru- 
ments. 
