On a Stercoscojiic Method of PJiotogra2)hic Surveying 29 



and if we put y=f, we get 











x,^ 



J,x 













~ a 



1^ 





Similarly 



the 



equation oi B P 



is 











X- 



-h 



V •2' - 



-h 





X- 



-b~ 



r z- 



-li 



where b and Ji are the x 



and 



z cc 



>-ordinates of B 



Whence 















X,=-y{X-h) + h 



From these equations we find 



X.. 



e is the stereoscopic dijference, constant for points in any plane 

 perpendicular to A y and vanishing for points at infinity. 

 The values of the co-ordinates of P follow 



b 



xJx^ 

 c " 



e ' 

 A check is afforded b}' the values of X and Z derived from B P, 



X= x^, — o 

 e 



zJ/..-h, 



x'u and z\ denoting here the co-ordinates of Q' referred to B. 



The measurement of the co-ordinates of a point being made in- 

 dependently on each plate, although simultaneously, it will be a 

 sufficient condition for the viewing apparatus to make corresponding, 

 portions of the two pictures combine with or without change of 

 perspective. 



Using a magnifying optical system to view the pair of plates, the 

 condition for distinct vision is that the two images of any point 

 appear in a corresponding plane of vision, so that the visual rays 

 meet in space. This condition evidently remains satisfied when the 

 images are magnified, or when they are brought nearer together 



