[ ] 
Fig- f- 
In this the greateft difficulty confifts in getting two 
well-center’ d objed-glafles, whofe focal lengths are 
equal ; for it is neceflary they ffiould be fo, becaufe 
they are to be combined with the fame convex eye- 
lens (common to them) at the fame diltance. a b is 
the diameter of a plain brafs plate, which may be two 
inches and a half broad, or fomewhat lefs ; two ffiort 
equal cylindric brafs tubes, m /?, r s , muft be fattened 
thereon, with their centers p c , equidiftant from the 
center / of the plate, and diftant one inch from one 
another in the diameter a b, as the figure fheweth. 
In the tubes muft be put two equal objed glaftes of 
the focal length of ioqd- inches, or rather fomewhat 
lefs, as aforefaid. Through the plate there muft be 
made in the middle of each tube a round aperture, 
viz. h g, w x, whofe diameters muft be proportion’d 
to the focal length of the eye-lens, and not exceed the 
third part thereof, left the objed appear confufed. 
And fince it is fcarce poflible to center an objed- 
lens to very good perfection, thofe in the two cylin- 
ders (tho’ put in by a good artift) may happen to ren- 
der the two folar images at too great a diftance from, 
or too near to one another. But this fault, if not 
too great, may be remedied, by turning one or both 
of the lens’s a little way round ; and then their eccen- 
tric poles will by that means be brought nearer to, 
or further from, one another ; and when they arc 
once well plac’d, there ftiould be a mark made in 
each lens, and its cylinder ; that if it is taken out to 
be wiped, it may be put in again the fame way. 
There ftiould alfo be a different mark in one of 
6 the 
