February 3, 19 16] 



NATURE 



621 



the rifle, and a pai-liole or orlhoptic mounted near 

 the breech ; the focal length of the lens is greater 

 than the distance between it and the pin-hole. 

 Figs. I and 2 show one of the arrangements em- 

 ployed. The whole sight weighed about three 

 ounces, and there was the minimum of apparatus 

 to get out of order. The line of aim is providcTi 

 by the line joining the pin-hole and a small dot 

 ground on the lens at the optical centre, the lens 

 being edged so that the spot is also the geo- 



FiG. 2. — Pin-hole or orihopiic ne.ir breach enJ. 



metrical centre in order to provide against rota- 

 tion of the lens in its mount; the orthoptic can be 

 elevated or deflected as shown, and thus any line 

 of aim suitable for a rifle can be attained. The 

 sight gives a magnification of about three times. 

 In introducing this sight Dr. Common offered 

 a prize in the Bisley meetings of 1902 and 1903, 

 the service rifle to be used, the range being 1000 

 yards, and the optical sight to be so mounted 

 that it did not interfere with the ordinary open 

 sights of the rifle. Some good shooting resulted, 



\,/000. Ydj. Flac^ jB/St,£.Y 



Fk,. 3. — Hii-. by a. tyro using Dr. Common's optical si^ht. 



and as a proof of the efficiency of the sight Fig. 3 

 represents the hits by the writer, who had never 

 previously shot at the butts. This sight received 

 some theoretical criticism as regards its use by 

 "myopes," but by practical trials with lenses of 

 suitable focal length, two riflemen with eye cor- 

 rections of —2 diopters were able to improve 

 their shooting considerably. To make the sight 

 suitable for different visions, the Birmingham 

 Small Arms Co. made an improvement by adding 

 NO. 2414, VOL. 96] 



a negative lens to the orthoptic, thus converting 

 the sight into a Galilean telescope ; this improves 

 the definition of the target, and since vision is 

 made through the concave lens and a peep-hole, 



Fig. 4. — Orthoptic and negative lens back sight for us 

 sight, as made by the U.S.A. Co. 



% 



with optlc.-xl fore 



the definition of the mark Qn the lens is not 

 seriously impaired. 



Coming to coUimating sights, the earliest, that 

 by Sir Howard Grubb, was described in Nature 

 of January 9, 1902. The following is a quotation 

 from that article : — 



By means of the sight a virtual image of a small 

 bright cross or circle is projected on the object aimed 

 at. The oarliest form in which the sight was made 



Fig. 5. — Sir Howard Grubb's collimating sight. 



is shown in Fig. 5, in which the object aimed at is 

 viewed through a tube open at each end, a piece of 

 parallel glass, PP, being fixed at an angle of 45° to the 

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

 the former a diaphragm i is fixed, made of an opaque 



