( 4^35 ) 
wherewith the Atmofphere is irapregnaled, corrodes and 
rufts it ; or at leaft , at its exhaliog^ leaves it covered o- 
ver with a thin skin^ confifting partly of an earthly fe- 
diment of that moifturej and partly of the duft ^ which 
flying to and fro in the Air had fetled and adhered to 
it. 
When there is not occafion to make frequent life of 
the inftrument , there may be other waies to preferve 
the metal for a long tinrie ; as perhaps by immerging 
it in Spirit of wine or fome other convenient liquor* 
And if they chancy to tarnifli 5 yet their polifh may be 
recovered by r^b'^ing ihem with a fofc piece of leather, 
or other tender fubftance, without the affiftance of any 
fretting powders , unle/s they happen to be rufty for 
then they muft be new poliflied. 
I am very fenfible^ that metal reflefls lefs light than 
glafs tranfmits ; and for that inconvenience, I gave you 
a remedy in my laft Letter 3 by afligning a fliallower 
charge ^n proportion to the Aperture, than is ufed in o- 
ther Telefcopes/ But, as I have found fome metaline 
fubftances to be more ftrongly reflediive, and to polilh 
better , and be freer from tarniihing than others 5 fo I 
hope there may in time. be. found out fome fubftance 
much freer from thefe inconveAiences ^ tbaia my yet 
known* 
OifemMioms 
