a Large Refracting Telescope. 325 



the same optical properties under all temperatures to which it 

 is likely to be exposed in astronomical observations, except per- 

 haps direct observations on the solar disc, which will probably 

 be found inadmissible. I felt so confident, from the result ob- 

 tained with the dispersive instrument, of the applicability of this 

 splendid fluid to the purposes I had in view, that after some 

 trials as to the best method of inclosing it, and of applying the 

 correcting lens, I attempted at once a telescope of six inches 

 aperture and of seven feet length; but some unforeseen difficulty 

 having interposed, after several unsuccessful trials, I laid it by, 

 and undertook one of three inches aperture. I was here more 

 fortunate, having, with this instrument in its first rude experi- 

 mental form, without any adaptation or selection of glasses, se- 

 parated a great number of double stars of that class which Sir 

 William Herschell has pointed out as tests of a good three and 

 a half inch refractor ; I can see with it the small star in Polaris 

 with a power of 46, and with the higher powers several stars 

 which are considered to require a good telescope, as, for example, 

 70, p Ophiuchi, 39 Bootis, the quadruple star i Lyrae, ^ Aquarii, 

 « HercuUs, ^c Encouraged by my success on this instrument, 

 I again attempted the six inch object glass, with a different man- 

 ner of adjusting and securing the lenses; and the result of my 

 endeavours I lay with confidence before the Board of Longi- 

 tude, feeling convinced that every proper allowance will be made 

 for the imperfections of a first attempt, at a novel construction, 

 on a considerable scale, and which professes only to prove the ap- 

 plicability of the principle, and not the completion of the expe- 

 riment. With this instrument the small star in Polaris is so 

 distinct and brilliant with a power of 1 43, that its transit might 

 be taken with the utmost certainty. But as this and the former 

 instruments are both before the Board of Longitude, and have 

 been examined by some of its members, I would much rather 

 they would report their opinion of the performance of them, 

 and more particularly of the promise they hold out; than to give 

 my own. I shall therefore proceed at once to describe the prin- 

 ciple of the proposed construction, which possesses some novel- 

 ty, and offers some advantage not to be obtained with any glass 

 ever made, or likely to be made ; although I am quite ready to 

 admit, that if glass could be obtained of sufficient purity and 

 size, the permanent nature of that material would, probably, 



JANUARY — MAECH 1828. Y 



