Grubb — A New Collimating-Telescope Gun-Sight for large and small Ordnance. 325 



At right angles to this tube is mounted a smaller tube carrying at its outer 

 end a diaphragm (rf), 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, near its 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 light from the sky or any other source of light which pass through the trans- 

 parent portion of the diaphragm, diverge until they reach this object-glass (o), 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. 



Fig. 



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 



