290 Mr. Herschel on Reflecting Telescopes. 



by metallic reflection, are, I think, somewhat too strong. He ob- 

 serves that, " the most perfect metallic mirror reflects only a 

 small part of the incident light, and that the greater part is ab- 

 sorbed ;" and that, " in consequence, the intensity of the light 

 entering the eye of the observer is always very small " (ist immer 

 sehrgering.) A metallic mirror, however, reflects 0.673 of the in- 

 cident light, or more than two-thirds, and absorbs less than one- 

 third of the whole. Mr. Fraunhofer appears rather to have had 

 in view the Newtonian construction, where two metallic mirrors 

 are used, and where the whole effective quantity of light is only 

 0.452 of the incident rays. No one who has been half blinded by 

 the entrance of Sirius or a Lyrae into one of my father's 20 feet 

 reflectors, will say that the intensity of its light is small, nor, to 

 take a less extreme case, will any one who uses one of M. Amici's 

 Newtonian reflectors of 12 inches aperture (a perfectly convenient 

 and manageable size, and of which he has constructed several,) 

 be disposed to complain of its want of light. The ordinary 

 reflector used by my father in his reviews of the Heavens was a 

 Newtonian, of 7 feet focus, and barely six inches in aperture, and 

 consequently equal (cceteris paribus) to an achromatic of 4j 

 (4.254) English, or 3.99 Paris inches, and therefore by no means 

 proper to be put in competition with Mr. Fraunhofer's chef- 

 d'ceuvres of 7 and 9 inches. Yet it will be recollected, that with 

 this telescope, and with a magnifying power of 460, u Leonis was 

 discovered to be double and distinctly separated, and its angle of 

 position measured. 



In order to demonstrate the superiority of refracting over re- 

 flecting telescopes, Mr. Fraunhofer has selected the star f Bootis, 

 which my father has described as a double star of the 6th class 

 (No. 104) in his second catalogue of double stars, but without 

 mentioning the division of the large star into two, as a double star 

 of the first class. It might, however, be very easily overlooked in 

 a review in indifferent weather. It is at least as difficult to re- 

 solve as v Coronas, more so than <r, either of which, with any tele- 

 scope, be its goodness what it may, requires a favourable atmo- 

 sphere for its separation. From this omission, however, Mr. 



