[ 17 ° ] 
at the time of his perigee than one inch in diameter> 
or half an inch radius. 
Fig. 2. 
The whole circle reprefents a well-center’d objed:- 
lens, whofe focal length is (as above calculated) 
104..96 inches and parts (rather a little lefs, that the 
two images may be fure not to touch one another.) 
Let the two diameters d m , qf divide it into four 
quadrants, but the diameter q f mud be occult, or 
delible. Let c w be half an inch, and c v equal to 
it. Through v (and alfo thro’ iv) let a chord -line 
be drawn parallel to the diameter dm, viz. b g, bp. 
Thro’ the faid chord-lines bg , and bp, and alfo thro’ 
the diameter d m, divide the lens into four parts. 
Fig. 3. 
Let the ftrait edge of the fruftum bvgq in the 
preceding figure be cemented fall to that of the fimi- 
lar fruftum bwpf of the fame lens, as they appear 
in this prefent Fig. 3. wherein, for the eafier under- 
ftanding the placing them, I have noted each fruftum 
with the fame letters it was noted withal in the pre- 
ceding Fig. 2. Having then with barm faftened a 
white paper all over both fides of the lens I made 
for trial (which I did, not only to fecure the cemented 
joint from breaking, but to prevent the injury which 
the polifh might receive in cutting and grinding the 
edges) I defcribed a circle q m nj on the center c , fit 
for the tube I had to put in it ; and having made it 
round, and waflfid it clean, after the edges were 
ground true, that nothing fandy might hurt the polifh, 
I foak’d it Jn clean water, till I could cafily take off 
