of Achromatic and Reflecting Telescopes. 313 



resolve it, and must, therefore, have been inferior to that of 

 an achromatic in the hands of Mr Bessel, with which it was 

 recognized by that eminent astronomer as double. It will 

 be seen, in reference to the Memoir on double stars by Mr 

 South and myself, that this star had been long since ascer- 

 tained to be double, not only by Mr Bessel, but by Messrs 

 Struve, Pond, and South, and, what is more to the present 

 purpose, by Sir W. Herschel himself. It was only by over- 

 sight that we omitted to refer, in that work, to his account 

 of it, which is published in his paper " on the places of 145 

 new double stars," in the first Vol. of the Transactions of the 

 Astronomical Society, p. 178, and read in June 1821. 



In large reflectors, in which only one metallic mirror is 

 used, the disadvantage, in point of light, under which they 

 labour, in comparison with refractors, is, however, much less 

 formidable. A reflector of eighteen inches aperture would 

 be equivalent to an achromatic of 15|, and one of 48 

 inches to an achromatic of 41^ in aperture, a size we cannot 

 suppose, (from any thing we have yet seen,) that it is possi- 

 ble the latter should ever attain. Reflectors of eighteen or 

 twenty inches are perfectly manageable, and, I apprehend, 

 quite within the power of any good artist to execute, and (if 

 intended only for use, and not at all for show) at no very 

 ruinous expence. That which I habitually use, of the for- 

 mer dimension, is my own workmanship, and though inferior 

 in distinctness to the exquisite one used by my father in his 

 sweeps, is by no means an instrument to be despised. In- 

 deed, from the experience I have had of these telescopes, I 

 am satisfied of their applicability, even to the more exact pur- 

 poses of astronomy, and that great improvements in their 

 construction and mechanism remain to be made. 



Having succeeded in observing the eclipses of Jupiter's Sa- 

 tellites in the day-time, and in measuring the diameters with 

 one of his Newtonian reflectors, Professor Amici was desi- 

 rous of ascertaining the dimensions of an achromatic telescope, 

 capable of showing these phenomena with the same distinct- 

 ness. 



" With this view,' 1 says he, " I took a Newtonian telescope 

 of my own making, having a focal distance of 30 inches, and 



