1902.] A Portable Telemeter, or Bang e- finder. 



349 



inclined at about 45° to each other. It emerges at the surface AE, 

 and suffers similar deflection by the second prism, and, if the angle 

 between the silvered surfaces BC, DE be the same in both prisms, and 

 if the axes of the prisms (i.e., the line of contact of reflecting surfaces) 

 be parallel, then the emerging ray must be parallel to the incident ray. 

 If the axes of the two prisms be not exactly parallel, a rotation of one 

 prism relatively to the other in the plane of vision, i.e., in a plane 

 approximately perpendicular to their axes, produces no error ; and a 

 rotation about any other axis, through any angle, produces an error 

 which is only a small fraction of that angle. Consequently no abso- 

 lute rigidity in the base connecting the first and second prisms is 

 required, as is the case when there is only a single reflection at each 

 end of the base. 



Y Qfeet *J 



FiGL 1. Path of rays of light through base prism to binocular of range-finder. 



(4.) Advantage of placing the Object Glass outside of and behind the Base. 

 — In all three instruments previously perfected the object glasses are 

 close to the outer prism, This necessitates the rigidity of the connec- 

 tion between the two halves of the base as well as between the 

 reflectors constituting each half. If one-half of the base were not 

 exactly in line with the other half, the image formed by it would be 

 displaced relatively to the other image. 



But when the base has no lens in it to form an image, if the reflect- 

 ing surfaces be parallel then the emergent rays which enter the tele- 

 scope must remain parallel to the incident ray, however much the 

 base as a whole be inclined in any direction. 



(5.) The Double Base and Hinge. — Even when the axes of the prisms 

 are not exactly parallel the two halves do not require to be absolutely 

 rigidly connected, and thus it has been possible to introduce a hinge 

 which doubles the portability of the base. One-half of the base may 

 be rotated with respect to the other about any axis. 



Rotation of one-half of the base, either about the line of vision or 

 about an axis perpendicular to the plane of vision, produces no effect 

 except that due to shortening the base by an amount that is immaterial 



2 B 2 



