Baotie.] -dli [May 6, 



sents on a map the actual visual angle as seen from that point of 

 view in nature. In fact, the latter truth is that which is in nature 

 the fundamental one in this connection. It is axiomatic that the 

 visual angles in nature between all objects whatsoever, as projected 

 on a given sector of the horizon, as seen by the eye of the 

 observer, or that of the camera, from a given point of view, are 

 the true angles between those objects, and that their sides, con- 

 verged at the point of view, represent the true directions of the 

 rays from those objects, corresponding with a base in nature with 

 reference to which their angles are either directly or indirectly, in 

 this case indirectly, known. Therefore it is because, in a single 

 picture, the angles between different objects, in fact between all 

 objects there, at the distance of the focal length of the camera, as 

 seen in tlie picture from its plotted point of view, are the same as 

 in nature from its point of view, that the intersection of rays from 

 the same object, as seen on different pictures, placed in position 

 corresponding with the way in which the landscape was photo- 

 graphed from nature, must represent by scale the horizontal posi- 

 tion of the object as it stands in nature. That is to say, if what 

 we see from one point of view in nature is true by angle, and also 

 by angle true, although different, as seen from another point of 

 view in nature, then the intersection of the individual rays, by 

 means of which we have seen the objects in their angular positions 

 with reference to each other, must represent their true horizontal 

 positions with reference to the base which we have traversed 

 between our respective points of view. And if this holds good 

 with respect to nature, it must hold good with respect to corre- 

 sponding pictures of nature, placed horizontally with relation to 

 each other as nature had presented itself from those individual 

 points of view from which the pictures were taken. The result, 

 expressed as a surveyor would state the case, depends upon the fact 

 that, if a point lies somewhere on a line, and also somewhere on a 

 line intersecting the other, then the point will be at the intersec- 

 tion of the two lines. In this case the two lines are simply the 

 visual rays, shown in the respective pictures, in the positions and 

 with the angular effects as seen in nature, intersecting each other 

 on their passage to the respective points of view. 



In practice, a round of pictures, each taking in a certain sector 

 of the horizon and intervening landscape, and slightly overlapping 

 one another, is made to cover the tract of which it is contemplated 



