r 175 ] 
and a whole inch from each other, let a circle for 
the aperture of each fruftum, of a proper lize, ac- 
cording to the intended charge of the telefcope, be 
defcribcd, and cut out. Alfo in the faid diameter, 
equidiftant from the center c 3 viz. at x and 2; , let 
there be a fcrew for each fruftum, to elevate it a 
little from the p'ate, as fhall be needful. 
Let there be a fpring contrived to prefs on the 
backfide of the one fruftum ora againft the point v, 
being the middle between the edge a 0 , and the fcrew 
x , to keep the fruftum clofe to the plate at the 
points a o 3 and alfo clofe to the fcrew x, when it 
is fcrew’d in. Let the like be alfo done on the back 
of the other fruftum esq. 
I fay then, 
1. That before the two fcrews are put in at x , 2;, 
the two fruftums of the mirror will lie plain on the 
plate of brafs, and have one pole at c common to 
them, and, confequently, will colled: all rays, which, 
during their incidence, are parallel to the axis of 
the tube to one common focus in the faid axis of 
the tube, juft as they would have done before the 
mirror was divided. 
2. But when the two fcrews a- 2; are put in their 
places, and fcrew’d a little way through the brafs 
plate, they will lift the two fruftums free from the 
plate at their circular edges, viz. at r and s 3 while 
their ftrait edges, a o 3 e q 3 are kept to touch the plate 
with both their ends (not in the middle, by reafon 
of the mirror’s concavity) by the prefture of the 
fprings, as was mention’d above. By this means the 
pole c of the fruftum ora will be removed from c 
toward 
