THE GREAT MELBOURNE TELESCOPE. 
159 
I regret that I had not access to any object-glasses by Fraunhofer * or Merz. Of the 
above, a belongs to the Armagh Observatory ; it is by one of the Dollonds, older than 
1790, and from its small proportional aperture, it is probably one of their first attempts 
at a triple combination. It is deficient in light, but defines very sharply, b. is the ori- 
ginal object-glass of the Armagh circle, extremely sharp ; but as it could not in this 
climate be depended on for observing stars below the 8-inch magnitude, it was replaced 
in 1861 by a remarkable one made by Mr. Grubb, of the same focus, but 7 inches aper- 
ture; it was made by Tulley about 1828; the crown is greenish, and I suppose 
English ; the flint was, I believe, from Daguet. I was much surprised at the great 
absorption of this glass, and therefore took several sets of measures ; but they all told 
the same story, c was made for me by Tulley in 1838 ; its glass is French, the crown is 
greenish, d is by Cauchoix. It came into my possession in 1837, but I have no infor- 
mation as to the maker of its glass ; the crown is greenish, and has probably a high n, but 
its mean thickness is only only 0*39. e is by Messrs. Cooke ; it had belonged to the late 
J udge Berwick, an excellent and accomplished man, who perished in the Abergele col- 
lision ; I do not know its date, but believe it recent ; the glass is Chance’s, f is by 
Grubb, the glass Chance’s : the very high transmission of this lens is in part due to 
the cementing of the adjacent surfaces, which, while it makes more difficult the correc- 
tion of spherical aberration, removes almost entirely the reflection at a surface of crown 
and one of flint: the factor for this = 0*9036 ; and if the I be multiplied by this, we 
obtain 0*7896, nearly that of e, the difference being due to the reflection at the film of 
cement, g is also by Grubb, and cemented ; its glass is by Chance. 
On examining this Table, the progressive increase in the .light of the object-glasses is 
evident. . The first two, which may be considered good specimens of the early achromatics 
have less illuminating-power than the Herschelian reflector. A great advance was made 
by Guinand and those who followed in his steps ; and a still greater one by Chance, 
whose glass is nearly perfect as to colour and transparency. 
The same inference follows from my measures of the index of absorption n , which, 
though not as numerous as I wish, are sufficiently significant. The specimens which I 
examined were, with two exceptions, prisms ; and this form is very convenient. If a ray 
is incident on an isosceles prism parallel to its base, it emerges parallel to itself after 
reflection at the base. If A be the angle of the prism and B, the refraction in this case, 
t the course of the ray in the prism = base X p can be found by one of the 
angles, and from it g 2 computed for an incidence = ^ A. I assume as the mean reflec- 
tion that due to the ^ of E ; it really is near b, but the greater illuminating-power of 
the yellow rays compensates for this. The due position of the prism is ensured by 
adjusting its base to the lines on the block already described. 
I give the results in Table II., in which are introduced those given in the paper that 
* March 9. — The President sent me his Fraunhofer, 3-2 aperture and about 45 focus. By 16 observations 
agreeing well, its coefficient =0*7393. 
MDCCCLXIX. 
Y 
