22 
indeed even with achromatics we find that sometimes a difh- 
cult object is best seen when the central part of it is covered. 
The reflection at 45° from metal is, in fact, a little fainter than 
the perpendicular one, from which incidence its intensity de- 
creases slowly till it becomes a minimum between 60° and 
70°. 
“¢ The advantages of the Cassegrain are :— 
“1. The tube is shorter. 
«<2, The observer is near the ground; he can easily be 
sheltered, and the eye-piece travels in a sphere of small ra- 
dius: while with the Newtonian he requires a complex appa- 
tus to support him, expensive and bulky. 
‘¢3. Any error of the large speculum can be corrected by 
figuring the small one to meet it. This, which was long since 
pointed out by Ramsden, is of much importance in large in- 
struments; for it is far easier, if the large speculum has 
not been perfectly figured, to let it abide, and work the other 
to it, having a few inches’ aperture, and weighing a few 
pounds, than to dismount and move one of a couple of 
tons,—the more so as as the control which is had over the 
process of polishing is very much greater in the first case. I 
say this from experience; for the great speculum of the Ar- 
magh was made parabolic, that it might be used as a Newto- 
nian, and the other was worked to correspond to it, as ‘while 
spherical it gave but an indifferent definition. 
‘4, Ifthe curvature of the second image be compared 
with the greater focal length of the ocular part which is re- 
quired to give the same magnifying power, it will be found 
that the field is flatter with the single lens, or Huygenian eye- 
pieces, than in the Newtonian. 
*¢5, The second image is so much larger than the primary 
one as to afford much facility for micromatic measurement. 
“©6, The adjustment of the specula is more easily verified. 
‘* These seem to me sufficient to invite the attention of 
any who are engaged in the construction of large telescopes to 
