A my J 
wherewith the Atmofphefe is impregnated, corrodes and 
rtifts it i or at leaft, at its exhaling, leaves if covered o- 
ver with a thin sklOj confiftiog "partly of an earthly fe- 
diment of that moifturCj and partly of the duft^ which 
flying to and fro in the Air had fetled and adhered to 
It. 
When there is not oecafion to make freqoent ufe of 
the inftrument, there maybe other waies to preferve 
the metal for a long time • as perhaps by immergiqg 
it in Spirit of wine or feme other convenient liqnor^ 
And if they chaneg to tarnifli ; yet their poliflh may be 
recovered by rubbing ihem witk 3. fofc piece of leather, 
or other tender (ubftance, without: the afiiftance of any 
fretting powders j unlefs they happen' to be rufty *• for 
then they muft be new polilhed. 
I am very fenfible, that metal reflefts lefs light than 
glafs tranlmitsi and for that inconvenience j I gave you 
a remedy in my laft Letter , by aflSgning a IhallowQr 
■ charge in proportion to the Aperture, than is ufedsin o- 
ther Telefcopes. ' But, as I have found fome metalioe 
fubft^nces to be more ftrongly refledivej and to polifh 
better , and be freer from tarnifliing than others 5 f® I 
hope there may in time be found out fome fubftance 
much freer from thefe inconveniences 3 than any y^C 
toow-n> 
