25 
and as a, the aperture of the lens, is, according to the usual 
practice of opticians, = 1 /’, we derive 
a( 62-763 + 4) — 0°18823.a? — 0:00394 a = F + = 
The proposed reflector is to have A = 4 feet; and if we make 
F'= 9A, which is Lord Rosse’s proportion ; we shall find 
a = 0°5135, or 6°16 inches. 
“From this follow f= 5-38, f° = 1:03, d=4-62, d’ = 32°74; 
the distance of mirrors = 31:38, M = 240; and @=6':92. The 
field of view is too small, but the arrangement may be conve- 
nient from the sharpness of vision obtained with the single 
lens. 
“¢ From what has been stated as to the power of correcting 
the figure of the large speculum by the small one, it does not 
seem necessary that F should be so great; in the Armagh te- 
lescope it is only 7:5A, and may be 8A. On this supposition 
we find 
a = 04641 = 5-57 inches. 
f = 4-24. 
f= 0-93. M = 240. 
d = 3-71. 0 = 625. 
d’ =29:59. 
Distance of mirror = 28°29. 
The Huyghenian eye-piece is much to be preferred in this 
case. Init 3f”=/', and the distance of the lenses = af’. 
This combination, it must be remembered, is not achromatic 
unless the rays emerge parallel, but where the lenses are so 
large as they are here, the colour is scarcely perceived. De- 
noting, as before, by ¢ the distance of the second image from 
the field-glass ; by u and z the distance of the two succeeding 
images from the lenses which form them; by w and z' those of 
the images of the small speculum, the last of which is at the 
eye-stop, we have 
