January 9, 1902] 



NA TUBE 



sight is placed in a geometric bracket for aiming and 

 removed immediately before the gun is fired, as other- 

 wise it would be injured by concussion. 



The gun-sight invented by Sir Howard Grubb, F.R.S., 

 is free from imperfections inherent in the old form of 

 telescope-sights used on rifles. The new instrument 

 is called by him the '' Collimating-telescope Gun-sight," 

 and a paper on the subject in the Transactions 

 of the Royal Dublin Society (March 20, 1901) at once 

 shows what considerations led up to the invention of the 

 new form of gun-sight. In it the inventor pictures an 

 ideal sighting arrangement thus : — He imagines a ring 

 or cross to be carried on a very long, weightless and 

 rigid rod forming a prolongation of the gun-barrel, so 

 that the ring or cross would always be situated in the 

 prolongation of the axis of the gun, and each shot, if 

 the trajectory were quite flat, would pass through the 

 ring. The inventor goes on to show that such an 

 ideal rod might be realised by using a fine beam 

 of lig'ht, which might be projected on to the object 

 and indicate the direction of the axis of the gun. 

 This end is in practice obtained by projecting a 

 •'virtual" image upon the object on which the gun is 

 aimed. By means of the gun-sight now to be described 

 a "virtual" image of a small bright cross or circle is 

 projected on to the object aimed at. The earliest form 



in which the gun-sight was made is shown in Fig. i, in 

 which the object aimed at is viewed through a piece of 

 tube of square section AH, open at each end, a piece of 

 parallel glass, /■/, being fixed at an angle of 45"^ to the axis 

 of the tube. In another tube at right angles to the 

 former a diaphragm d is fixed, made of glass coated with 

 an opaque substance, through which fine lines are 

 scratched in the pattern of a cross or star or circle, o is 

 an achromatic lens, and the distance between the cross and 

 the lens equals the principal focus of the lens : so that rays 

 of light passing through the cross, on reaching the lens, 

 are by it made parallel, they are then reflected by the 

 plates pp as parallel rays to the observer's eye, and the 

 observer sees a "virtual '' image of the cross coinciding 

 with the object aimed at, and apparently at the same dis- 

 tance as the object. This optical device causes the cross 

 to be seen sharply defined, with the same focussing of the 

 eye required for viewing the distant object, and all strain- 

 ing of the eye, as is the case in the old system, vanishes ; 

 also there is no parallax, and therefore the eye need not 

 be kept in one position. This " virtual " image of the 

 cross, forms a fore-sight projected to a long distance in 

 front of the rifle, as if it were carried upon an invisible, 



NO. 1680, VOL. 65] 



imponderable and inflexible prolongation of the barrel. 

 This form of apparatus not being of convenient shape 

 for practical use, the gun-sight eventually assumed the 

 form shown in Fig. 2, in which d is the cross, which is 

 reflected by the mirror c, on to a curved glass surface r>\ 

 coated with a very thin layer of galena, the ray is reflected 

 as a parallel beam to the eye, and at the same time the 



object is seen through the coated surface. The radius 

 of curvature of rr equals twice the distances dc and CE. 

 When the eye is placed anywhere near the axis of the 

 gun-sight the bright cross is seen superposed on the 

 object, and the usual effort required in the attempt to 

 focus two objects not at the same distance is entirely 

 avoided. The photograph (Fig. 3), taken by a camera 



placed behind the gun-sight and focussed on the dome of a 

 building, shows how perfectly the cross coincides with the 

 object, both the object and cross being in perfect focus. 



The gun-sight is mounted on a graduated metal arc 

 attached to the rifle, by means of which it is adjusted for 

 various ranges. The sight may be used either with or 

 without a telescope or inonocular, since the same focus 



