2 On Great Telescopes of the Future. 
over refractors become of greater value for the larger sized 
instruments. Among these the author makes mention of “absence 
of secondary spectrum,” “ possibility of equable support,” and 
“general convenience for observing purposes.” 
At the same time the author shows that there are many 
interesting physical problems to be solved for which it will be 
absolutely necessary to use reflectors of large size. 
The author then discusses the practical difficulties of con- - 
struction of each type of telescope, and the most promising means 
of overcoming these difficulties. 
The author argues that the practical limit to the increase of 
size of refractors is that of procuring suitable discs of glass. 
They have never yet been obtained over twenty-nine inches in 
diameter, and it is probable that in the present state of the art it 
would not be possible to procure them of many inches over that 
S1Ze. 
Supposing, however, that large discs were obtainable, a 
question arises whether the edge support, which is alone admis- 
sible in a refractor would not cause such a strain as would 
injure the performance of the objective, either by distortion of 
image or temporary polarization of the light. 
_ To obviate this, should it be found necessary, the author has 
devised an apparatus by which each disc of glass is partially 
supported on an air cushion ; air being forced into the telescope 
tube, and regulated in pressure by an automatic contrivance 
according to the altitude pro tempore of the telescope, (the lower 
end of the tube is supposed to be closed by a small lens equiva- 
lent to a low power Barlow.) 
Silver on glass mirrors the author places out of the discussion 
altogether as in the present state of the art of glass making it is 
not found possible to obtain discs over six feet in diameter. 
The great dithculty in the preparation of metallic reflectors is in 
the successful cooling of the disc, and to obviate this difficulty the 
author has devised a peculiar kind of ADA oven which he 
describes at length. 
With this oven it is possible (without of course opening bj in 
anyway) in the first place to ascertain exactly the relative heat 
of different parts of the disc of metal, and 2ndly to rer the 
heat of the different parts according to desire. 

