334 



ber of double stars of the class which Sir William Herschel has 

 pointed out as tests of a good 3|-inch refractor. Encouraged by his 

 success, he again attempted a 6-inch object-glass, with a different 

 manner of adjusting and securing the lenses, and considers the result 

 of his endeavour as proving at least the practicability of the construc- 

 tion. This instrument, with a power of 143, shows the small star in 

 Polaris so distinct and brilliant, that its transit might be taken with 

 the utmost certainty ; it exhibits distinctly the small stars in a Lyrae, 

 Aldebaran, Rigel, &c. and decidedly separates Castor, y Leonis, and 

 e Bootis. The belts and double ring of Saturn are well exhibited with 

 a power of 150 ; and the belts and satellites of Jupiter are tolerably 

 defined with the same power, but will not bear a higher power than 

 about 200. 



In the usual construction of achromatic telescopes, the two or the 

 three lenses composing the object-glass are brought into immediate 

 contact. But the high dispersive power of the sulphuret of carbon 

 enables Mr. Barlow to place the fluid correcting lens at a distance 

 from the plate object lens equal to half its focal length. By this 

 means the fluid lens, which is the most difficult part of the construc- 

 tion, is reduced to one half or less of the size of the plate lens. This 

 construction, therefore, renders us independent of flint glass, enables 

 us to increase the aperture of the telescope to a considerable extent ; 

 and gives us all the light, field, and focal power of a telescope of one 

 and a half time the length of the tube. The author investigates 

 analytically the formulae for calculating the proper distance of the 

 lenses on this construction, and expresses a hope that further experi- 

 ments will enable us to determine the precise distance which shall 

 reduce what has been termed the secondary spectrum, inseparable 

 from the ordinary construction, either to zero, or to an inconsiderable 

 amount. 



A Catalogue of Nebulae and clusters of Stars in the Southern Hemi- 

 sphere, observed at Paramatta in New South Wales, by James Dun- 

 lop, Esq. In a Letter addressed to Sir Thomas Makdougall Bris- 

 bane, Bart. K.C.B. late Governor of New South Wales. Presented 

 to the Royal Society by John Frederick William Herschel, Esq. Vice 

 President. Read December 20, 1827. [Phil. Trans. 1828, p. 113.] 



The observations, of which the results are here given, were made 

 by Mr. Dunlop in the open air, with a 9 -feet reflecting telescope, 

 having the clear aperture of the large mirror 9 inches, and fitted up 

 as a meridian telescope ; the position of which, and the index error, 

 being ascertained by the passage of known stars. The drawings 

 which accompany the paper were made at the time of observation 

 of the appearances of a great number of nebulae and clusters, and 

 particularly of the nebulae major and minor. The paper contains a 

 catalogue and description of 629 nebulae, arranged in the order of 

 their south polar distance, and in zones for each degree in the order 

 of their right ascension. A few observations are subjoined, descri- 

 bing more particularly the appearance of the nebula minor, which, 



