Bache.] '^^^ [May 6, 



The rays of light eA and cB, respective!}-, proceed from a steeple 

 to the photographic points of view A and B. The intersection of 

 these rays at 6" gives, according to the scale of the plotted base, the 

 horizontal position by scale of the steeple with reference to those 

 points of view. 



In deference to a misunderstanding of which I heard lately, as 

 to the relations of the horizons to each other, as represented in this 

 diagram, I shall endeavor to preclude it now by calling attention 

 to the fact that the horizons are here laid down as to direction, but 

 in order to secure the utmost simplicity in the diagram, not as to 

 their possibility of lateral extension. Objects, for instance, in the 

 middle distance of the steeple, as seen in elevation on horizon A, 

 from station A, would, at station B, fall to the left of the steeple, 

 as seen in elevation on horizon B. Conversely, objects in the gen- 

 eral direction, and in the background of the steeple, as seen in the 

 elevation on horizon A, from station A, would fall to the right of 

 the steeple, as seen in elevation on horizon B, from station B. 



The chief method of photogrammetry in use at the present time 

 is illustrated by this diagram, and is based on very simple princi- 

 ples. The angles and distances obtained in ordinary surveying are 

 merely natural or artificial selections. They are merely arbitrary 

 subdivisions of space, convenient selections from an infinite num- 

 ber of similar elements. But it is also true that, the relations of a 

 few of those elements being judiciously selected and determined, 

 all others secondarily deduced fall into harmony with them. The 

 photographic camera, however, as compared with other surveying 

 instruments, does not lend itself at first to selection, but giving all 

 visible nature from various points of view, enables the employer of 

 it finally to make his selection from the resulting pictures, as if from 

 nature itself. 



From all points of view, then, angles and distances exist in 

 nature, and although they apparently change, as the observer 

 changes place, the correspondences among them, as seen from all 

 points of view, are perfect. Hence, if we delegate to the photo- 

 graphic camera the duty of making a permanent record of nature, 

 as seen from two or more points of view, the intersection of the 

 rays of light, reaching those points of view respectively from the 

 same objects, as pictured in photographs properly placed, will, by 

 their intersection on paper to become a map, give the positions of 



