220 Professor Barlow on Achromatic Telescopes 



Art. IV. — Farther observations on the Construction of' Achro- 

 matic, Telescopes with a fluid concave lens, instead of the 

 usual lens of Flint Glass. * By Peter Barlow, Esq. 

 F. R S. &c. 



We have already in two successive papers, (No. xiv. p. 335, 

 and No. xv. p. 93,) made our readers acquainted with the im- 

 provements on the achromatic telescope proposed by Professor 

 Barlow ; and we are sure that they will partake in our gratifi- 

 cation when we inform them, that the Board of Longitude has 

 authorized him to carry on his experiments at their expence. 



Since the date of Professor Barlow's last communication to 

 this Journal, he has had several opportunities of applying his 

 telescope with six inches aperture and seven feet in length to 

 the examination of double stars. 



" With this instrument," says he, " the small star in Polaris 

 is so distinct and brilliant with a power of 143, that its transit 

 might be taken with the utmost certainty. The small stars in 

 a Lyrae, Aldebaran, Rigel, s Bootis, &c. are very distinctly ex- 

 hibited ; amongst the larger close double stars, Castor and y 

 Leonis are well defined with a power of 300 ; and amongst the 

 smaller double stars I may mention w Aurigse, 52 Orionis, £ 

 Ononis, and a variety of others of the same class. The belts 

 and double ring of Saturn are well exhibited with a power of 

 150 ; and the belts and satellites of Jupiter are very tolerably 

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

 than about 200, in his present situation, which is certainly not 

 favourable : in both cases the discs of the planets are satisfac- 

 torily white, and belts and shadows well marked ; but in Ju- 

 piter, and perhaps in both, there is some uncorrected colour 

 round the edge of the disc.'" 



" Besides the advantages attending this principle of construc- 

 tion, which were formerly pointed out, I am willing to hope 

 that another very important one (which I have not, however, 

 been able at present practically to demonstrate) may still be 

 effected ; namely, the reduction of what has been termed the 



• The paper of which this is an abstract was read before the Koyal So- 

 ciety of London on the 17th January 1828. Ed. 



