Lord Oxmantown on a new Reflechng Telescope. 25 



Art. II. — Account of a New Reflecting Telescope. By the 

 Right Honourable Lord Oxmantown, M. P. &c. Com- 

 municated by the Author. 



The following considerations induced me to make the expe- 

 riments on the specula of reflecting telescopes which I am 

 about to describe. The reflecting telescope would be almost 

 a perfect instrument, could we devise means of freeing it from 

 spherical aberration ; it would then retain merely the defects 

 necessarily arising from imperfections in the workmanship, and 

 perhaps some others of a much more trifling nature, such as 

 those derived from the inflection of light. The refractor, 

 however, is not only affected by the spherical aberration in 

 common with the reflector, but also by the different refrangibi- 

 lity of the rays of light. Both of these defects may indeed be 

 in a great degree corrected by giving curves of proper radii to 

 the lenses which compose the object-glass. The spherical aber- 

 ration may by this be almost entirely obliterated, but a consi- 

 derable portion of the chromatic aberration still remains, owing 

 to the irrationality of the spectra formed by the different kinds 

 of glass, of which the object-glass is necessarily composed, the 

 different coloured rays not being refracted by each in the same 

 proportion. The refractors until lately were limited to a very 

 small scale, owing to the impossibility of procuring suitable 

 glass of large dimensions; and although a new process has 

 lately been discovered on the continent, which has considerably 

 extended the limits of their construction, still I believe that 

 large pieces of glass, of a tolerably homogeneous nature, are 

 procured with great difficulty ; and there seems to be but little 

 prospect of our being able, with the present state of our know- 

 ledge, to construct efficient refractors, at leas^ with glass lenses 

 of apertures at all approaching the late Sir William Herschel's 

 reflectors. I have been thus minute in stating my reasons for 

 making the following experiments, as many practical men 

 whom I have spoken to seem to think that since Fraunhofers 

 discoveries, the refractor has entirely superseded the reflec- 

 tor, and that all attempts to improve the latter instrument are 

 useless. 



