An Extract of another Letter of the same to the Publisher, dated March 30. 1672. by way of Answer to some Objections, made by an Ingenious French Philosopher to the New Reflecting Telescope.


I Doubt not but M.A. will allow the advantage of reflexion 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 practique part, it is in some measure manifest by the Instruments already made, to what degree of vivacity and brightness a metaline substance may be polished. Nor is it improbable but that there may be new ways of polishing found out for metal, which will far excell 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, <4035> 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 earthly sediment of that moisture, and partly of the dust, which flying to and fro in the Air had setled and adhered to it.

When there is not occasion to make frequent use of the instrument, there may be other waies to preserve the metal for a long time; as perhaps by immerging it in Spirit of wine or some other convenient liquor. And if they chance to tarnish; yet their polish may be recovered by rubbing them with a soft piece of leather, or other tender substance, without the assistance of any fretting powers, unless they happen to be rusty: for then they must be new polished.

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 shallower charge in proportion to the Aperture, than is used in other Telescopes. But, as I have found some metaline substances to be more strongly reflective, and to polish better, and be freer from tarnishing than others; so I hope there may in time be found out some substance much freer from these inconveniences, than any yet known.

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