toward r ; and likewife the pole e of the other fruf- 
tam esq be removed from c towards s , more or lefs 
according to the quantity of the elevation of each 
frulium by the fcrew that raifeth it : fo that now there 
will appear at the focus two folar images j whereas 
there was but one, before the fcrews were put in. 
By moving the fcrews, the two folar images may 
be brought to any diftance from one another ; but 
care mu ft be taken not to raife one fruftum more 
than the other, and the two folar images mull almoft; 
touch one another at the time of the perigee ; other- 
wife it muft be better adjufted. 
This telefcope may be finiflied with a fmall el- 
liptical fpecillum of black glafs, ground plain on its 
refteding furface, and a convex eye-lens, like that 
def ribed by J. Hadley, Efqj F. R. S. in Phil. Tranf. 
N° 37 6. A micrometer may be contrived for it at 
the common focus, near the eye-lens. 
Such a double objed-fpeculum would be capable 
of a vaft improvement, by combining it with a con- 
cave fpecillum, which would reflect the images thro’ 
a hole in the center c of the faid fpeculum to fall on 
a convex eye-lens, after the manner of our new fort 
of rerledting telefcopes, was it not for the difficulty of 
adapting fuch a micrometer to it as would exadly 
meafure minutes and feconds ; for the eye-glaftes of 
fuch having ulually a pretty large focal length, would 
bear much larger divifions on a micrometer, than 
Pvdr. Hadley’s with a fmall eye- glafs can do, tho’ 
their charges lliould be equal, or that of the former 
did exceed. 
I find that large objed-glaftes for telefcopes are not 
commonly well center’d, with their poles in the very- 
middle 
