292 Mr. Herschel on Reflecting Telescopes. 



A reflector of 18 inches aperture would be equivalent to an 

 achromatic of 15£, and one of 48 inches to an achromatic of 

 41 J in aperture, a size we cannot suppose (from any thing we 

 have yet seen) that it is passible the latter should ever attain. 

 Reflectors of 18 or 20 inches are perfectly manageable, and, I ap- 

 prehend, 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 expense. That which I habitually use, of the former 

 dimension, is my own workmanship, (en amateur), and though in- 

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

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

 from the experience I have had of these telescopes, I am satisfied 

 of their applicability even to the more exact purposes of astro- 

 nomy, and that great improvements in their construction and 

 mechanism remain to be made. 



Having referred to the paper on double stars, published by Mr. 

 South and myself, permit me (with his concurrence) to rectify 

 some errors into which we have there fallen, and which have been 

 recognised by him in the course of his observations at Passy in 

 continuation of the same subject, the results of which, I hope, 

 will, ere long, be before the public, and will afford a convincing 

 proof of his zeal and indefatigable industry. — The first of these 

 errors is in the case of the small star accompanying 36 Ophiuchi. 

 It is stated in the work referred to (as the result of a single 

 measure, probably a hasty one, or affected by an accidental de- 

 rangement of the micrometer head, as having its position 19° 5' 

 np) and distance 3' 0".735. There is no memorandum of which of 

 the close stars the position was measured from, but it must have 

 been from the most southern, as Mr. South, by a mean of 10 

 measures, finds 17° 41' np for its angle of position with respect to 

 this star. The difference (l° 24') is not very important, but the 

 distance is greatly in error. Mr. S. has determined the distance 

 of the small star from the more southern of the close stars (by a 

 mean of 19 measures) at 3' 15".252, and from the more northern 

 (by a mean of 21) at 3' 13".689. 



The next correction I have to notice is in the case of it Aquilse, 



