666 
MEMORIAL DE ARTILLERlA. 
very close to the eye, such as the tangent scale and the sight on the 
gun, in size proportionate to that of the object glass of the telescope. 
We can with this optical apparatus, lay as correctly as when simply 
looking over the sights with the naked eye at short ranges, and no 
special mechanism is needed to bring the optical axis into exact 
relation with the different lines of sight of guns. 
The telescope is worked by hand, bringing the object glass against 
the sight, or else by placing it on a support on the breech of the 
gun; which latter is the more convenient way. 
This valuable effect is obtained by employing a convex and achro¬ 
matic half lens, cut exactly in the centre, which intercepts the passage 
of the luminous rays. 
