VOL. VII.] PHILOSOPHICAL TRANSACTIONS. 705 



the edges. And the thickness of the metal must be proportional to the breadtli, 

 lest it bend in the grinding. The metals being polished, there may be trials 

 made with several eye-glasses, to find what charge may with best advantage be 

 made use of. 



An Extract of another Letter of the same to the Editor, dated March 

 30, 1672, by way of Ansiver to some Objections made by an inge- 

 nious French Philosopher to the New Reflecting Telescope. N" 82, 

 p. 4034. 



Sir, — I doubt not but M.A. will allow the advantage of reflection in the 

 theory to be very great, when he shall have informed himself of the different 

 refrangibility of the several rays of light. And for the practical part, it is in 

 some measure manifest by the instruments already made, to what degree of vi- 

 vacity and brightness a metalline substance may be polished. Nor is it impro- 

 bable but that there may be new ways of polishing found out for metal, which 

 will far excel those that are yet in use. And when a metal is once well polished, 

 it will be a long while preserved from tarnishing, if diligence be used to keep 

 it dry and close shut up from air; for the principal cause of tarnishing seems to 

 be the condensing of moisture on its polished surface, which by an acid spirit, 

 wherewith the atmosphere is impregnated, corrodes and rusts it; or at least, at 

 its exhaling, leaves it covered over with a thin skin, consisting partly of an 

 earthy sediment of that moisture, and partly of the dust, which, flying to and 

 fro in the air, had settled and adhered to it. 



When there is not occasion to make frequent use of the instrument, there 

 may be other ways to preserve the metal for a long time: as perhaps by im- 

 merging it in spirit of wine or some other convenient liquor. And if it 

 chance to tarnish, yet its polish may be recovered by rubbing it with a soft 

 piece of leather, or other tender substance, without the assistance of any 

 fretting powders, unless it happen to be rusty, for then it must be new po- 

 lished. 



I am very sensible that metal reflects less light than glass transmits, and for 

 that inconvenience I gave you a remedy in my last letter, by assigning a shal- 

 lower charge in proportion to the aperture, than is used in other telescopes. 

 But, as I have found some metalline substances to be more strongly reflective, 

 and to polish better, and be more free from tarnishing than others, so I hope 

 there may in time be found out some substance much freer from these inconve- 

 niences than any yet known. 



VOL. I. 4 U 



