118 Mr. Airy on the use of Silvered Glass, §c. 
large aperture, the parabolic form is absolutely necessary, and 
this can be given only by the best workmen. The advantage 
of this construction in which none but spherical surfaces are used 
is sufticiently obvious. The difference in the quantity of light 
reflected, if there is any, appears so small that it offers a very 
slight objection; especially as we can increase the aperture with- 
out fear of indistinctness for aberration. 
I have constructed two Cassegrain’s telescopes on this plan, 
whose object-mirrors are 4 inches in aperture, and 20 inches in 
focal length. From some cause with which [ am unacquainted 
the image of a star or planet is surrounded with radiations which 
make the telescope quite useless for practical purposes, and render 
it extremely difficult to pronounce any thing on the success of 
the principle. I have not however been able to observe the 
slightest appearance of colour: of the spherical aberration I can- 
not, in consequence of the radiation, speak so decidedly, but I am 
certain that if there is any it is very small. But the success of 
a principle of this kind is not to be determined from one or two 
experiments; several trials should be made, and every endeavour 
used to overcome the difficulties which always occur in instru- 
ments made on a new construction; and even if it should appear 
at present to fail, some improvement in the theoretical principles 
or in the practical application may make it useful hereafter. 
It is my intention to make new trials; and to attempt the cor- 
rection of the defect at present existing; and the results of my 
experiments will be communicated to the Society should this 
paper appear worthy of their attention. 
G. B. AIRY. 
Trinity CoLuece, 
Nov. 25, 1822. 
