60 The Earl of Rosse’s Telescopes, and their 
any competition. For comparing the space-penetrating power of 
the six-feet speculum with one of two feet (which has rarely been 
exceeded) we find it three to one in favor of the largest, with an 
accumulation of light in the ratio of 6? to 27, or 9to 1. Oncom- 
paring the powers of this magnificent instrument with those of a 
refractor of two feet aperture, the largest hitherto attempted, we 
have a superiority—making a due allowance for the loss of light 
by reflection from two mirrors, and assuming an equal degree of 
perfectness, figure, and other optical requirements in the refractor, 
and no allowance for absorption of light—in the ratio of about 
4-5 to 1, as to light, and as 2-12 to 1,as to the capability of pene- 
trating space, or detecting nebulous or sidereal objects at the ex- 
treme distance of visibility. Hence, whilst the range of telesco- 
pic vision in a refractor of two feet aperture would embrace @ 
sphere in space represented by.a diameter of 2; the six-feet spect 
lum (assuming both instruments to be of equal optical perfec- 
tion, magnifying equally, and allowing fifty per cent. for loss of 
light for two reflections in the one case, and none (?) in the other) 
would comprehend a sphere of about 4:24 diameter,—the outet 
shell of which, 1:12 in thickness, being the province of the great 
instrumentalone. But let us reduce these proportions to sections 
of equal spaces, that we may judge more accurately of the rela- 
tive powers. Now, the solid contents of different spheres, We 
know, are in the ratio of the cubes of their diameters. Hence 
the comparative spheres, penetrated by the two instruments Te 
referred to, should be 4°24? to 2; that is, as 9'5to lL. Deducting; 
then, from this vast grasp of space the inner sphere, capable © 
being explored by other instruments, we find that, out of nearly 
ten sections of space reached by this telescope, there are nearly 
nine sections which the six feet speculum may embrace as pect 
liarly its own! 
What its revelations yet may prove, then, we can have no idea. 
Several thousands of nebule have been catalogued: the great re 
at the Cape of Good Hope, for the examination of the heavens 
towards the southern pole. Lord Rosse, with his usual noble- 
ness of liberality, will yield up his laboratory, machinery, and 
men, to the service of government, and is willing, moreover, 1 
give the direction and guidance of his master-mind. Will the 
British nation be content with a refusal ? 
The range opened to us by the great telescope at Birr Castle, 
is best, perhaps, apprehended by the now usual measurement— 
- notof distances in miles, or millions of miles, or diameters of the 
