147 



author's object is, first to present, under a general and uniform ana- 

 lysis, the whole theory of the aberration of spherical surfaces ; and 

 then to furnish practical results of easy computation to the artist, 

 and applicable, by the simplest interpolations, to the ordinary ma- 

 terials on which he works. In pursuing these ends he has found it 

 necessary somewhat to alter the usual language employed by optical 

 writers ; thus, instead of speaking of the focal length of lenses, or the 

 radii of their surfaces, he speaks of their powers and curvatures ; de- 

 signating, by the former expression, the quotient of unity by the 

 number of parts of any scale which the focal length is equal to; 

 and by the latter, the quotient similarly derived from the radius in 

 question. 



After adverting to some other parts of the subject of this paper, 

 more especially to the problem of the destruction of the spherical 

 aberration in a double or multiple lens, and to the difficulties which 

 it involves, Mr. Herschel observes, that one condition, hitherto un- 

 accountably overlooked, is forced upon our attention by the nature 

 of the formulae of aberration given in this paper ; namely, its de- 

 struction not only from parallel rays, but also from rays diverging 

 from a point at any finite distance, and which is required in a perfect 

 telescope for land objects, and is of considerable advantage in those 

 for astronomical use : 1st, The very moderate curvatures required 

 for the surfaces ; 2nd, That in this construction the curvatures of 

 the two exterior surfaces of the compound lens of given focal length 

 vary within very narrow limits, by any variation in either the refrac- 

 tive or dispersive powers at all likely to occur in practice ; 3rd, 

 That the two interior surfaces always approach so nearly to coinci- 

 dence, that no considerable practical error can arise from neglecting 

 their difference, and figuring them on tools of equal radii. 



An Account of the Skeletons of the Dugong > Two-horned Rhinoceros, 

 and Tapir of Sumatra, sent to England by Sir Thomas Stamford 

 Raffles, Governor of Bencoolen. By Sir Everard Home, Bart. 

 V.P.R.S. Read March 22, 1821. [Phil. Trans. 1821, p. 268.] 



In this paper, Sir Everard first describes, by reference to an an- 

 nexed drawing, the peculiar form of the skeleton of the dugong, 

 which he compares to a boat without a keel, with the bottom upper- 

 most ; so that in the sea the middle of the back is the highest point 

 in the water ; and as the lungs are very extensive, they render the 

 animal buoyant. As a compensation for legs, the dugong has c pe- 

 culiar means of suspending itself in the sea, the centre of the back 

 forming the point of suspension, similar to the fulcrum of a pair of 

 scales ; this peculiarity explains the form of the jaws, which are 

 placed at an angle with the skull, unlike those of any other animal. 



There is no remarkable difference between the bones of the two- 

 horned rhinoceros, compared with those of the single-horned species, 

 except that the projection in front of the skull, formed by the union 

 of the nasal bones, is more nearly in a straight line and more ex- 



T. 2 



