C 5 6 ° ] 
Now let this be applied to the fubjed in hand. 
Suppofe the focal diftance of the divided objed-glafs 
to be about forty feet ; and fuppofe the fegments to 
be open’d wide enough to bring the oppofite edges of 
an objed in contad : Then let another objed-glafs, 
uncut, be fix’d within the tube, of a proper degree of 
convexity, to fhcrten the focus of the other as much 
as may be requir’d ; fuppofe to twelve feet : By what 
has been juft now obferv’d, this glafs will rcprefent 
the two images in the fame form which would have 
been exhibited by the divided glafs, if this other glafs 
had not been there. For tho’ the images are not yet 
form’d, when the fecond glafs receives the rays ; yet, 
as thofe rays are converging towards it, the fecond 
glafs muft reprefent thofe images in the fame pofition, 
and form, as the tendency of the rays requires. For 
while the fegments are fix’d in their pofition to each 
other, their images will alfo be fix’d in their pofition; 
and let them be repeated ever fo many times, by re- 
fraction thro’ fpherical glades, or by reflexion from 
fpherical mirrors, they can differ no alteration in their 
pofition to one another. By this means, the telefcope 
may be jfhorten’d, at plealuie, tho’ the fcale for the 
meafure of the angles will remain the fame. The 
only inconvenience, which the fhortnefs of the tele- 
fcope introduces, is a want of fufEcient diftindnefs ; 
which will fo far hinder the exadnefs of the obser- 
vation, as the contad of the edges cannot be fo accu- 
rately detei min'd, as they might be with longer tele- 
lcopes. 
This difficulty is intirely remov’d by fixing the di- 
vided glafs at the end of a refleding telefcope : For 
the reflexions and refradions, which the rays mud 
undergo 
