( 4^34 ) 
In like manner^ if a half foot Tclefcope may diftinftly mag- 
nifie 36 limes with i~ of an Inch Aperture ^ a four foot Te- 
lefcopcfliould with equal diftinitnclsmrtgnific 171 times with 
6 inches Aperture 3 and one of fix foot ihould inagnifie 252 
times with 8f inches Aperture ; and fo of other lengths. But 
what the event will really be, we nuift wait ro 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 Inftruments. Thus for a four 
foot Tube, fincc the Aperture (hould be 5 or 6 inches, there 
will be req[uired a piece of metal 7 or 8 inches broad at leaft, 
becaufe 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 poliflbed , 
there may be tryals made with feveral eye?g!affes , to find, 
what Charge may with beft advantage be made ufe 
of. 
An ExtraB of another Letter of the fame to the Publijher ^ dated 
March 30. 1672, byway of Anfwer to fmeOhjeSionr^ made 
by an Ingenious French Philofopher to^ the New RejleHing Tehf 
40pe^ 
^ SIR, 
I Doubt not but M.A. will allow the advantage of reflexioa 
in the Theory to be very greatj, when he fliall have infor- 
tned himfelf of the different Rejrangibility of the feveral rays 
of light. And for the pra^lique part, it is in fome meafure 
fnanifeft by the Inftruments already made ,^ to what degree 
of vivacity and brightnefs a metaline fubftancc may be po- 
lilhed. Nor is it improbable but that there may be new ways 
of polifhing found out for metal, which will fiir excel! thofe 
that arc yet in ufc. And when a metal is once well poHftied, 
it will be along while prefervedfromtarnifhing^ if diligence 
Ijc ufed to keep it dry and clofe, (hut up from Air: For the 
principal cwfe of tarnifliingfccms to be, thccondenfing ®f 
moiftarc onitspolifhed furface, which by an Acid fpirit, 
where- 
