Royal Society. 45 



efficacy of telescopes constructed with fluid lenses, on the princi- 

 ples developed in his two former papers, published in the Phil. 

 Trans., in separating double stars, resolving nebulae, and exhibiting 

 different appearances in the discs of the planets. He institutes, with 

 this view, a comparison between the performance of his telescope 

 of 8 inches aperture and 12 feet in length, with Mr. Herschel's 

 telescope, made with his new 20 inches speculum, and with Sir 

 James South's new refractor, of 12 inches aperture and 20 feet fo- 

 cal length. In Mr. Barlow's telescope 7; Persei, which is marked as 

 double in South and Herschel's catalogue, is seen distinctly sextuple. 

 The stars composing a Orionis, marked in the catalogue as two dis- 

 tinct sets of stars, each triple, are shown in Mr. Barlow's telescope as 

 being both quadruple, with two very fine stars between them. A 

 very fine double star was discovered by Mr. Herschel between the 

 two which compose /3 Capricorni, and was considered by him as a 

 very severe test : this star is seen distinctly in Mr. Barlow's tele- 

 scope, but not double. 



Messier's 22nd nebula is resolved by Sir James South's tele- 

 scope into an immense number of brilliant small stars. In Mr. Bar- 

 low's telescope the same resolution is effected, though somewhat 

 less completely. 



The two last-mentioned instances he considers as affording ex- 

 cellent criteria of the exact limits of the power of the instrument. 



Mr. Barlow next examined Jupiter and Mars in order to com- 

 pare the defining powers of the two instruments. Both these planets 

 were more sharply defined in Sir James South's telescope than in 

 that of the author, but in this respect the superiority of the former 

 instrument was by no means great : and in the exhibition of the 

 shadow of one of Jupiter's satellites passing over his disc, there 

 was no apparent difference between the two instruments. Their 

 powers seemed as nearly as possible equal when applied to Mars. 



An experience of three years has not shown the slightest per- 

 ceptible change in either the quantity or quality of the fluid em- 

 ployed as the lens of the author's three-inch telescope ; neither has 

 the glass inclosing it suffered any diminution of its transparency. 

 The author conceives it therefore to be sufficiently established, 

 that sulphuret of carbon is capable of supplying all the properties 

 of flint-glass, which are required in the construction of a telescope; 

 and moreover, that in consequence of its high dispersive power, it 

 admits of being placed so far behind the principal lens of plate- or 

 crown-glass, as to require to be only one half of the diameter of the 

 latter. This combination also gives a focal power of one and a 

 half times the length of the tube ; and consequently the telescope 

 may be reduced in length to two-thirds of that which a glass tele- 

 scope of the usual construction would require for an equal amount 

 of spherical aberration. In the conclusion of his paper, the author 

 proposes what he considers as a great improvement in the plan of 

 construction for very large telescopes on this principle : it consists 

 in making the object- lenses double, by which their spherical aber- 

 ration may at once be reduced to about one-fourth of its present 



amount 



