Grupp—A New Collimating-Telescope Gun-Sight for large and small Ordnance. 326 
At right angles to this tube is mounted a smaller tube carrying at its outer 
end a diaphragm (d), preferably of glass coated with some opaque material, through 
which lines are cut representing a cross, star, circle, or any other desired 
device. 
At the base of this same tube, nearits junction with the main or sighting tube, 
is placed an achromatic lens, the distance between the diaphragm and the 
achromatic lens being equal to the principal focus of that lens; consequently rays 
of hight from the sky or any other source of ight which pass through the trans- 
parent portion of the diaphragm, diverge until they reach this object-glass (0), and 
are by it rendered parallel, and are reflected by the diagonal plate or plates, pp, 
once again as parallel rays, into the eye of the observer; the result being that the 
observer sees, superposed upon the object he is aiming at, an image (generally 
called a ‘‘ virtual” image), of the cross or device cut upon the diaphragm; and 
inasmuch as the arrangement, when properly adjusted, is such that the rays 
from the diaphragm enter the eye under exactly the same conditions as if from 
the distant object, the cross appears not only superposed on the object, but at the 
same distance as the object itself. As a consequence of this, the cross is seen 
absolutely sharp with the same focussing of the eye as that necessary for viewing 
the distant object, and there is no straining of the eye to see both in focus at the 
same time; also it follows that there is no parallax, that is to say, that the cross 
and the object aimed at, if made to coincide when the eye is in the centre of the 
tube, will equally well coincide no matter what portion of the sighting tube the eye 
is placed opposite to; in other words, there is no necessity for the observer to 
keep his eye in any fixed position. 
In fact this “virtual” image of the cross forms a fore-sight to the gun 
projected at a long distance in front of the barrel itself, as if it were carried 
