[ r 7§ J 
oppofite fide ; for I could plainly obferve a difference 
afterward, if I negle&ed to mind both thefe circum- 
flances, or indeed either of them *. 
XXVII. A Defcription of a Contrivance for 
meafuring fmall Angles, by Mr, John- 
Dollond ; communicated by Mr, J. Short,, 
F, R, S, 
Head May io, T ET an objedt-glafs, of any conveni- 
I j ent focal length (being truly ground 
and well center’d) be divided into two equal parts 
or fegments, by cutting it fhrait through the center ; 
and let a piece of machinery be fo contriv’d, as to 
hold thefe two fegments in the fame pofition to 
each other, as they flood in before they were cut 
afunderj and to be capable at the fame time of 
drawing them to different didances from that po^ 
lition, in the manner, as is reprefented in the figure. 
Each of thefe fegments will form a diflindt image- 
of any objedt, to which they are directed';, differing 
in nothing from that, which might have been made 
by the whole glafs before it was cut, except in bright- 
nefs. And while thefe fegments are held in their 
original pofition, the images will coincide, and become 
one fingle image as at firfl^but, in proportion as 
they 
— — 
* Dr. Smith, in his Complete Syftem of Optics, publifhed in 
1738, has deferibed a very accurate and ready method of cen;er- 
ing objedt-glaflcs, which was always ufed by the late Mr. George 
Graham, from whom the do&or had it. 
