622 



NATURE 



[February 3, 19 16 



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 focal length of the 

 lens; so that rays of light passing through the cross, 

 on reaching the lens, are by it made parallel ; they 



Fig. 6. — Grubb's sight aligned on a distant object. 



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 distance as the object. 

 This optical device causes the cross to be seen sharply 

 defined, with the same focusing of the eye required 

 for viewing the distant object, and all straining 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 

 foresight projected to a long distance 

 in front of the rifle, as if it were car- 

 ried upon an invisible, imponderable, 

 and inflexible prolongation of the 

 barrel. 



The optical arrangement of the 

 sig-ht was afterwards modified to 

 make it convenient for mounting- on 

 a graduated arc attached to the 

 rifle, but its principle remained the 

 same. Grubb's sight may be used 

 with or without a telescope, since 

 the same focus suits both the object 

 and the image of the cross ; also by 

 cutting divided scales on the dia- 

 phragm glass, useful estimates 

 may be made of both distance and 

 windage. 



Fig. 6 shows a photograph taken 

 by a camera placed behind the sight 

 and focused on a distant object ; 

 both the fiducial cross of the sight 

 and the distant object are seen to be in perfect 

 focus. 



Several other collimating sights followed, the 



simplest, apparently originating in France, 



though patented by Krupp in England, is one 



in which a long Stanhope lens is used, the lens 



NO. 2414, VOL. 96] 



having a V-shaped channel cut along its entire 

 length, and the apex of the channel being the 

 axis of the lens. A sight may thus be taken, 

 along the groove, whilst at the same time a por- 

 tion of the pupil of the eye catches parallel rays 

 issuing from the lens, on the flat remote face of 

 which is a fiducial mark. 



Mr. Dennis Taylor, of the firm of Messrs. Cooke 

 and Sons, York, took out a patent in 1901, in 

 which use was made of a Galilean telescope to 

 which was attached a collimator; the upper half 

 of the eye-lens of the telescope was cut away and 

 a prism substituted in its place. The function of 

 the prism was to direct the emergent beam from 

 the collimator into juxtaposition with the emergent 

 beam from the telescope, so that both beams were 

 visible at the same time. Dr. Common and others 

 used this half-eye or half-pupil arrangement for 

 viewing two objects at the same time, but none met 

 with success, chiefly because the average person 

 experiences great difficulty in adjusting the eye 

 with that nicety required to catch the two beams 

 at the same time. 



To obviate the difficulties arising from the half- 

 pupil arrangement. Common employed the prin- 

 ciple of the collimating sight in other ways ; 

 in a patent of 1901, he used a small collimator 

 with its mark at the principal focus of the lens, 

 hence, looking into the sight, the mark appeared 

 at infinity. If, now, both eyes remained open, 

 one might be used to look at the object aimed 

 at, whilst the other eye looked into the collimator ; 

 the fiducial mark could then be superposed on the 

 target. This sigl;it would answer perfectly well 



Fig. 7. — Dr. Common shooting with telescope rifle sight in " Winans" conipetitiou, Bisley, 1902. 



if fusion of the different visual fields could be 

 obtained, but when the eyes are used in this way, 

 "antagonism of the visual field" occurs^ and the 

 whole or part of one of the fields may be sup- 

 pressed and the sight becomes useless. 



1 Tscherning, " Physiological Optics," p. 323. 



