316 Dr. Robinson on the Great Melbourne Telescope. [Mar. 4, 



absorption, n. The specimens 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 undergoing total 

 internal reflection at the base ; and the length of the path of the light 

 within the glass, and the loss by the two reflections, are easily calculated 

 from the known angle and refractive index. The mean index used in the 

 calculation was that of the line E. 



The results are given in the following Table, in which are introduced 

 those given in the paper that they may be referred to at once ; and there is 

 added to them one found in Bouguer's 'Traite d'Optique/ which seems 



trustworthy. 



Description. n 



1. Prism, originally Captain Kater's 0*1829 



2. French plate, Mr. Grubb 0-1728 



3. London plate, Mr. Grubb 0-2140 



4. Two of same, Mr. Grubb 0-1446 



5. Prism, Mr. Grubb 0*061/ 



6. Bouguer's glass 0-1895 



7. Gassiot's prisms 0-6209 



8. Prism by Dubosq, flint 0'1504 



9. Prism by Merz, flint 0-1089 



10. Prism by Merz, crown 0*0858 



11. Prism by Merz, flint 0*1065 



12. Prism by Grubb 0*0218 



13. Cylinder of crown 0'0272 



14. Cylinder of flint 0-0090 



No. 1 was shown to the author 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. 



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 has probably not varied in compo- 

 sition, and its p. has been used in the computation. 



No. 7 consists of two prisms of 60°, which Mr. Gassiot, with his wonted 

 kindness, intrusted to the author for some inquiries about the improvement 

 of the spectroscope. They are by Merz, of glass which seems nearly iden- 

 tical with Faraday's dense glass, having a specific gravity of 5*1, and a 

 mean fi = 1-7664. It is very pellucid, but, like its prototype, has a yellowish 

 tinge, probably given by the large proportion of lead. As Merz does not 

 polish the base or ends of his prisms, the usual method could not be 

 employed ; but the prisms were put together with the angles opposed, and 

 a drop of olive-oil between, and the reflections allowed for. The great 



