Grupp—A New Collimating-Telescope Gun-Sight for large and small Ordnance. 327 
placed in a small hood or projection at the top of the 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 coneave 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 the 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 the 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 any definite position so long 
as if is near enough to the axis to see the +; all that is necessary is to centre 
the + on the object, and it does not matter if, in doing this, the + 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. 
