312 Mr Herschel on the Relative Performances 



a small part of the incident light, and that the greater part 

 is absorbed ;' and that, in consequence, the intensity of the 

 light entering the eye of the observer, is always very small. " 

 A metallic mirror, however, reflects 0.673 of the incident 

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

 third of the whole. M. Fraunhofer appears rather to have 

 in -view the Newtonian construction, where two metallic mir- 

 rors 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 of a Lyrae, into 

 one of my father's twenty feet reflectors, will say that the in- 

 tensity of its light is small, nor, to take a less extreme case, 

 will any one who uses one of M. Amici's Newtonian reflec- 

 tors of twelve 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 re- 

 flector used by my father in his reviews of the Heavens, was 

 a Newtonian, seven feet focus, and barely six inches in aper- 

 ture ; and, consequently, equal, ceteris paribus, to an achro- 

 matic of 4| (4.254) English, or 3.999 Paris inches, and there- 

 fore by no means proper to be put in competition with M. 

 Fraunhofer's chef cTceuvres of seven and nine inches; yet it will 

 be recollected, that, with this telescope, and with a magnify- 

 ing power of 460, w Leonis was discovered to be double, and 

 distinctly separated, and its angle of position measured. 



In order to demonstrate the superiority of refracting over 

 reflecting telescopes, M. Fraunhofer has selected the star 

 £ Bootis, which my father has described as a double star of the 

 sixth 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, how- 

 ever, be very easily overlooked in a review in indifferent 

 weather. It is, at least, as difficult to resolve as i Coronas, 

 more so than <r, either of which, with any telescope, be its 

 goodness what it may, requires a favourable atmosphere for 

 its separation. From this omission, however, Mr. Fraunhofer 

 concludes, that the power of the telescope was insufficient to 



" The experiments on which this result is founded, are detailed in the 

 A> tide Optics in the Edinburgh Encyclopedia, vol. xv., p. 625, 626. — En. 



