A 4034 ) . 
Iq like manner, if a half foot Telefcope may diftinfily mag- 
nifie 36 times with i| of an Inch Aperture 5 a four foot Te- 
lefcope fliould with equal diftinftnefsmagnific 171 times with 
f inches Aperture 3 and one of fix foot ihculd piagnifie 252 
times with 8f inches Aperture ^ and fo of other lengths. But 
what the event will really be, we muft wait to fee determined 
by experience. Only this I thought fit to infinuate, that they 
which intend to make trials in other lengths^ may more rea- 
dily know how to defign their loftrumeuts. Thus for a four 
foot TubCj fince the Aperture fliouId be 5 or 6 imchesj there 
will be required a piece of metal 7 or 8 inches broad at leaft^ 
becaufc the figure will fcarcely be true to the edges. And the 
thicknefsof the metal muft be proportional to the breadth, 
leaft it bend in the grinding. The metalls being poIiCbed 5, 
there may be tryals made with feveral eye=gla{Fes , to find, 
what Charge may with beft advantage be made ufe 
of. 
dn ExtraB ef anotier Letter of the fame to the Tuhlijher\ datect 
March 30. 1672. byway of Anfwer to fffmeObjeUionij made 
by an Ingenious Ftench Pbilofopher to the New RefieUing Telef 
SI Rj 
I Doubt not but M.A. will allow the advantage of reflexioi 
in the Theory to be very great, when he ftiall have infor- 
med himfelf oi thc&xStt^ntReJrangibility of the feveral rays 
of light. And for the praftique part , it is in fome mcafure 
manifeft by the Inftrumcnts already made ^ to what degree 
of vivacity and brightnefs a metaline fubftance may be po- 
liced. Nor is it improbable but that there may be new ways 
of polifliing found out for metal^ which will far excell thofe 
that ar« yet in ufc. And when a metal is once well poliflied, 
it will be a long while preferved from tarnifliing, if diligence 
be^ ufed to keep it dry and clofe^ fliut up from Air : For the 
prsncipal caufe of tarnifliing feems to be, thecondenfing of 
moifture on its poliflied furfaec , whick by an Acid Ipirit, 
where- 
