THE G-REA.T MELBOURNE TELESCOPE. 
161 
No. 10, of 90°, was obtained by the late Lord Rosse to be similarly used in his 3-feet 
Newtonian. Like me, however, he was disappointed. Its base— 3*05, and its p for 
E=l-5321. 
No. 11, of 60°, obtained by me at Munich in 1837. For these measures I had the 
ends polished flat; its for E=l - 6405. 
These three show considerable progress, and an object-glass made of such materials 
would have a great power of transmission, though much behind the following. 
No. 12 is of 90°. Its glass is from Chance; its base=2-65, and its p for E=l-6216. 
No. 13 is a cylinder 2*2 inches in diameter and 4'3 long, which Mr. Grubb obtained 
from Messrs. Chance for these measures; its p for E=l-5200. 
No. 14 is a cylinder got at the same time, 2T inches in diameter and 4*4 long; its g. 
for E = 1 -6126 ; the ends of both are polished flat, and they are of wonderful transparency. 
I was nearly as much surprised at the very low value of n in these three last speci- 
mens, as I was at the great absorption of the object-glasses a and b in Table I. But 
the following considerations will show that these measures, especially in the two last, 
must be very near the truth. I find from 110 observations, in which 6 ranged from 
29° 23' to 75° 27', that the probable error of a single determination of I by the 
Zollner= + 0'0251. As dl=db X sin 23, it may be inferred that the probable error of 
the single observations for the I of No. 14 = 0-0157. Again, dn= ; and the pro- 
bable error of this n~ + 0-0019, only a fifth of its actual value, so that its correctness 
has a high probability. This is confirmed by the values of I in the object-glasses e,f, 
and g, which would not be possible if n was much larger. For instance, if it were=0T, 
the I of g would be 0-7585 instead of 0-8408. 
If, as I see good ground for hoping, Messrs. Chance shall succeed in manufacturing 
large disks of the same perfection as these two cylinders, my comparison of the achro- 
matic and the reflector must be considerably modified. I think we may assume n= 0-02 
as the highest excellence likely to be attained on such a scale ; and if this be intro- 
duced into the expression given in the paper for the intensity of the achromatic, it 
becomes 
I=log _1 (9-90964)xe- AxloB_1(7 ' 46482) . 
If this be multiplied by A 2 and equated to 0-401 X48 2 , the quantity of light trans- 
mitted by a 4-feet Newtonian, we have an equation which when solved gives 35 -435 for 
the aperture of an equivalent achromatic. This aperture would be diminished if the 
process of cementing were found applicable to lenses of such magnitude ; but if such an 
object-glass were ever attempted, its focus would probably be much shorter than eighteen 
times its aperture, and therefore its increased thickness would produce a contrary effect. 
I shall conclude with suggesting that, as very slight variations in the manufacture of 
glass seem to make great changes in its absorptive power, it would be prudent to exa- 
mine the value of n in the disks intended for lenses of any importance. This could 
be done by polishing a couple of facets on their edges, and need not involve the sacrifice 
of many minutes. 
