6o8 



NATURE 



[October i8, 1900 



years in working out the details of the instrument, it will 

 probably be known in the future by their name. In the tele- 

 stereoscope of Helmholtz two parallel reflectors are placed with 

 reference to each eye, in such a position as to produce the optical 

 equivalent of an increase in distance between the two eyes. 



Then when we look into the instrument at a distant landscape 

 we shall get the appearance shown in Fig. i. If this picture 

 be viewed stereoscopically we shall obtain a mental impression 

 as to the distance of any part of the landscape by comparison 

 with the marks, and in this way get its approximate range. 



Fig. I.— Field of view in the Grousillier-Zeiss stereoscopic range-finder. 



This increases the stereoscopic effect. In the next place, by 

 placing a telescope before each eye (and the two telescopes may 

 conveniently be incorporated in the frame- piece supporting the 

 reflectors), we multiply the stereoscopic effect still further, e.g. 

 if the distance between the eyes has been artificially increased 

 tenfold by the reflectors, and if the telescopes magnify tenfold, 

 the stereoscopic effect will be increased altogether one hundred- 

 fold. In this way immense stereoscopic solidity is imparted to 

 the picture in the field of view. 



Fig. 2.— Bridge of H.M.S Royal Soverfign, showing Barr and Stroud 

 range-finder on the top of the chart-house. 



This instrument may be adapted to range-finding in the follow- 

 ing way -. — Suppose we imagine for the moment the instrument 

 fixed, and that we see in the field of view of each eye the image 

 of a pole 1000 yards away. Let permanent marks be made in 

 the focal plane of each telescope exactly coincident with these 

 images. Then, whenever we look into the instrument we shall 

 apparently see a pole at a distance of looo yards. Let a similar 

 pair of marks be fixed corresponding to i lOO yards, and so on. 



NO. 1616 VOL. 62] 



The instrument is highly ingenious and very pretty, and it will 

 no doubt offer a solution of the problem of military range- 

 finding should it prove sufiiciently accurate in practice. 



{4) B. and S. Naval Range-fivder. — This instrument, with 

 a base of 4^ feet, has been adopted in her Majesty's Navy, and 



Fig. 3, 



d Stroud range-finder on fortress mounting. 



nearly all the larger ships are now equipped with one or more 

 of them. Fig. 2 shows a view of the charthouse of H.M.S. 

 Royal Sovereign, on the top of which the range-finder will be 

 seen near the Kelvin compass. 



