160 
DR. T. R. ROBINSON AND MR. T. GRUBB’S DESCRIPTION OF 
they may be referred to at once ; and I add to them one which I found in Bouguer’s 
“ Traite d’Optique,” which seems trustworthy. 
Description. 
e 2 - 
t. 
I. 
n . 
Obs. 
1. Prism, originally Captain Kater's... 
1 r 
1-125 
0-746 
0-1829 
5 
2. French plate, Mr. Grubb 
0-75 
0-805 
0-1728 
3. London plate, Mr. Grubb 
>0-9164 > 
0-30 
0-86 
0-2140 
4. Two of same, Mr. Grubb 
f 1 
0-60 
0-77 
0-1446 
5. Prism, Mr. Grubb 
J l 
2-00 
0-81 
0-0617 
6. Bouguer’s glass 
0-9109 
3-200 
0-500 
0-1895 
7. Gassiot’s prisms 
f 0-8380 1 
1 0-9763 j 
1-915 
0-2490 
0-6209 
5 
3. Prism by Dubosq, flint 
0-8878 
1-730 
0-6844 
0-1504 
12 
9. Prism by Merz, flint 
0-8647 
1-580 
0-7549 
01089 
5 
10. Prism by Merz, crown 
0-8935 
2-431 
0-7253 
0-0858 
5 
11. Prism by Merz, flint 
0-8854 
1-500 
0-7550 
0-1065 
24 
12. Prism by Grubb 
0-8696 
2-721 
0-8196 
0-0218 
6 
13. Cylinder of crown 
0-9166 
4-300 
0-8155 
0-0272 
12 
14. Cylinder of flint 
0-8907 
4-400 
0-8563 
0-0090 
6 
No. 1 was shown to me in 1830 by Captain Kater, as the chef-d'oeuvre of the Glass- 
Committee ; he used it as the small speculum of his Newtonian. Afterwards it came 
into the possession of the late Lord Rosse, who made the above measures with Bunsen’s 
photometer in 1848. It is English plate, greenish. It is curious that its n should be 
so near No. 6, a century before. 
Nos. 2, 3, 4, 5 were measured by Mr. Grubb in 1857. No. 5 was a prism of 90°. He 
does not remember its history ; but evidently it was of Chance’s glass. 
No. 6 is described by Bouguer as “ glace,” 3 Paris inches thick. It was probably that 
of St. Gobain, which I suppose has not varied in composition, and I have used its p in 
computing f. 
No. 7 is two prisms of 60°, which Mr. Gassiot, with his wonted kindness, entrusted 
to me for some inquiries about the improvement of the spectroscope. They are by Merz, 
of glass which seems nearly identical with Faraday’s dense glass, having a specific gravity 
of 5L, and a mean ^=1-7664. It is very pellucid, but, like its prototype, has a yellowish 
tinge, which I suppose is given by the large proportion of lead. As Merz does not 
polish the base or ends of his prisms, I could not use my usual method, but I put them 
together with the angles opposed, and a drop of olive-oil between*. The incidence could 
not be perpendicular, as there was a partial reflection at the oil, but a full beam passed 
at an incidence of 30°. The great absorption is remarkable, and cannot, I think, be 
explained by the colour of the glass. The side of the largest of these prisms=2 , 60. 
No. 8 is of 60°; its side=l'90, and its p for E=l , 6200. It is free from colour, and 
an evident improvement on the earlier ones. 
No. 9, a prism of 90°, was given to me by Dr. Lloyd for a small mirror in the New- 
tonian form of the Armagh 15-inch reflector. I was surprised that it had so little supe- 
riority over the metal one ; but the I explains this. A prism of No. 14 would have 
told a different storyf. Its base = 2-01, and its ^ for E = l - 6188. 
* The second f given is that of the film of oil. 
t With such a prism the coefficient of the Newtonian would he 0-548. 
