1902.] 



A Portable Telemeter, or Range-finder. 



347 



Other tests were instituted with the object of ascertaining whether 

 the illumination of the spark-gap of the transmitter had any effect 

 upon the impulses transmitted, and accordingly the ball dischargers 

 were inclosed in a box opaque to light. No perceptible difference, 

 however, was noticed in the strength of the signals received, whether 

 the spark-balls were or were not exposed to daylight. 



It would be interesting to ascertain whether the same effects are to 

 be observed when using transmitting elevated conductors covered with 

 insulating material opaque to ordinary light. 



I have never noticed any appreciable difference in the distances over 

 which signals are obtainable during the day and the night respectively 

 in the course of all the other numerous experiments which I have 

 carried out with installations not designed for very long distances, and 

 in which the electrical power used at the sending station has been small 

 compared with that used at the Poldhu installation. 



Probably the much higher potential to which the elevated conductor 

 at Poldhu was charged may have greatly increased the facility with 

 which losses might occur, due to diselectrification through the influence 

 of daylight. 



I hope to be able to make a complete study of the effects described 

 in this note, in the course of further long-distance tests which are 

 likely to be undertaken shortly. 



" A Portable Telemeter, or Range-finder." By Geokge Forbes, 

 F.R.S. Received February 22— Eead March 20, 1902. 



CONTENTS. 



1. General description. 



2. The Adie telemeter. 



3. Double reflection at each end of the base. 



4. Advantage of placing the object-glass outside of and behind the base. 



5. The double base and hinge. 



6. Accuracy required. 



7. The binocular. 



8. Advantages of stereoscopic vision. 



9. Construction and mounting of the prisms. 



10. Errors of the base and hinge. 



11. Errors of the binoculars. 



12. Practical adjustments. 



(1.) General Description, — The instrument consists of a folding steel 

 base, 6 feet in length, and a field glass. The base is a square tube hinged 

 at its middle, and folds up to 3 feet 3 inches. Each half has at each 

 end a doubly reflecting prism. The rays of light from a distant object 

 .strike the outer pair of these four prisms, are reflected at right 



vol,, lxx. 2 B 



