Grubb — A New Colliinating-Telescope Gun- Sight for large and synall Ordnance. 327 



placed in a small hood or projection at tlie top of tlie sighting tube, the divergent 

 rays from which diaphragm are reflected from the silvered portion of a piece of 

 plane glass which also forms the back window of the sighting tube, this, being 

 placed at a certain angle, reflects the still divergent rays on to the concave 

 surface of the front window, and these rays are by it rendered parallel and 

 reflected into the eye of the observer under exactly the same conditions as the 

 rays which enter his eye from the distant object. 



This front window is made concave on the inside, and convex on the 

 outside, so that there is no magnification or diminution in the apparent size of the 

 image, but the concave surface is coated with this reflective film, and tlie result 

 is the formation of a sufficiently brilliant image of the + superposed upon the 

 object. 



In using any of these various forms of sights, the effect is best described by saying 

 that on looking through the sight at the object, the latter is seen as through faintly 

 smoke-tinted glass, owing to the light having to pass through the semi-transparent 

 film, and when tlie eye takes up a position anywhere near the axis of the sight, a 

 bright +, star, or circle, or any other device adopted, is seen superposed on the 

 object. There is no necessity to keep the eye in an}^ definite position so long 

 as it is near enough to the axis to see the + ; all that is necessaiy is to centre 

 the -I- on the object, and it does not matter if, in doing this, the 4- be brought to 

 the centre of the field, or sides, or corners; the aiming will be equally good, and 

 there is the greatest possible ease and comfort in this operation. The + being as 

 distant as the object, both are seen distinctly without any of that teasing effect 

 due to the muscular effort in trying to focus simultaneously two objects that are 

 at different distances. 



In order to demonstrate the fact that this virtual image is formed at a long 

 distance in front of the gun, the two illustrations, figs. 4 and 5, are reproduced here 

 from photographs. These photographs were taken by a camera placed a few 

 feet behind the sight. 



In fig. 4, the camera was focussed on the body of the sight itself ; and it 

 will be seen that while the actual sighting tube, &c., is quite clear and sharp, 

 the distant view is completely out of focus, and there is no cross to be seen 

 as it also is quite out of focus. 



Fig. 5 was taken from exactly the same position, the camera not having been 

 altered in position, and under exactly the same conditions except that the lens 

 was focussed on the distant object instead of on the body of the sight, and as 

 will be seen, the latter is quite out of focus, but the distant object and the cross 

 are both quite sharp and clearly defined, proving that the virtual image of the 

 cross is formed at a considerable distance in front of the gun itself. 



