328 Mr Barlow's account of the Construction of 



AeT. XVIII. — An account of the preliminary experiments 

 and ultimate construction of a Refracting Telescope of 7.8 

 inches aperture^ with a fiuid concave lens.* By Peter 

 Barlow, Esq. F. R. S. &c. 



The instrument I intend more particularly to describe in this 

 paper has a clear aperture of 7.8 inches, exceeding, I think, 

 by about an inch the largest refracting telescope in this coun- 

 try. Its tube is eleven feet, which, together with the eye- 

 piece, makes the whole length twelve feet : but its effective 

 focus is, on the principle explained in my former paper,*)- 

 eighteen feet. It carries a power of 700 on the closest double 

 stars in South^s and HerschePs catalogue ; and the stars are 

 with that power round and defined, although the field is not 

 then so bright as I could desire. 



The telescope is mounted on a revolving stand, which 

 works with considerable accuracy as an azimuth and altitude 

 instrument. To give steadiness to the stand it has been made 

 substantial and heavy, its weight by estimation being 400 

 pounds, and that of the telescope 130 pounds ; yet its motions 

 are so smooth, and the power so arranged, that it may be ma- 

 naged by one person with the greatest ease, the star being fol- 

 lowed by a slight touch, scarcely exceeding that required for 

 the keys of a piano-forte. 



In my former paper {Phil, Trans. 1828 : Art. VII.) I have 

 endeavoured to show the effect which opening the lenses to 

 different distances produces on the secondary spectrum ; my 

 first object, therefore, in these experiments, was to ascertain 

 by actual observation the best position of the lenses for the 

 diminution of this defect. 



In order the better to classify my experiments on this head, 

 it will be best to refer to the original formula for the destruc- 

 tion of colour, given in my paper in the Phil. Trans. 1827 : 

 Art. XV. in which I have shown, that with open lenses we 



■'' Abridged from Phil. Trans. 1829, p. 32. See this Journal, No. xiv. 

 p. 335 ; No. xv. p. 93 ; and No. xviii. p. 220. 

 t Phil. Trans, 1828 : Art. vii. 



