C *7jf ] 
the glaftes, that each may know its own cylinder. 
They muft both of them be very clofe all round to 
their refpedtive cylinders ; otherwife one lens may 
hide nearer to or further from the other ; which if it 
fhould in the lead degree, between the firft and fe- 
cond obfervation, all the labour would be loft. 
Either of thefe three parts of double lens’s may be 
combined with a convex eye-lens as ufual, and have 
a micrometer placed at the common focus. 
Such a double lens, of either fort, may be proved 
whether it is well compofed or not, without the 
trouble of combining it with its eye-lens ; by holding 
it in the fun’s rays, as one would a burning glafs, 
and applying a piece of white paper at its focus, 
where, I apprehend, the two folar images will appear 
as diftindt as when an eye-lens is applied, tho’ not fo 
large ; and each of them one inch broad, if the fo- 
cal length is as above, /. e. almoft 104,7% inches. 
After the fame manner may the double objedt- 
mirror of a reflecting telefcope for this ufe be proved. 
Fig. 6. 
The circle bdhpmg is the circumference of a con- 
cave mirror made of black glafs : it muft be very 
thick, that it may not fpring or bend with any thing 
that prefleth on it to keep it faft j for that may in- 
jure the concavity of it. 
The circle within it on the fame center c fheweth, 
that the concavity thereof muft not be continued 
quite home to the very edge of the mirror, but the 
little fpace between the two circles muft be ground 
very true on a plain. The prick d lines muft not 
be drawn j they are only to indicate where the poles 
