1892.] -J^l [Bache. 



those objects relatively, as plotted, to the points of view and to one 

 another. 



Occupying with a photographic camera the points formed by the 

 termini of a base line on the surface of the earth, having on its 

 photographic plate imaginary vertical and horizontal lines, suscep- 

 tible of being developed into real ones, the intersection of these 

 lines corresponding with the centre of the prospective picture (the 

 former enabling the operator to set the camera accurately to any 

 horizontal direction, the latter giving, when the camera is leveled, 

 the horizon for each picture), the camera is fixed in turn at the two 

 stations upon some distant determinate object by its line of sight, 

 its position being otherwise so adjusted that the objects to be de- 

 termined in the landscape, within a given sector of the horizon, 

 shall appear on the picture as taken from each of the two stations. 

 The azimuth of the base line, and of the lines of sight from it, 

 being determined by the theodolite, field transit, or compass, the 

 survey for a particular sector of the horizon at the two stations 

 lacks but one factor to make it complete, as soon as the pictures 

 shall have been taken by the camera. The camera has given, by 

 its occupation of the two stations at the ends of the base line of 

 assumed length, only one portion of the data necessary to consti- 

 tuting a survey, namely, the angles subtended in nature by the vari- 

 ous objects which come within the scope of both resulting pictures. 

 A very simple addition, however, suffices to make the survey com- 

 plete. To secure that, to introduce the element of scale, it is 

 necessary to know the length of the base line. The scale to which 

 the base line is plotted on paper becomes, then, through the 

 acquisition of knowledge of the length of the base on the ground, 

 the scale of the whole resultant map; which, it should be inciden- 

 tally noted, must range by scale no further from each station than 

 to a distance where rays of light to the two stations give good 

 graphical intersection, the extent of the range by scale being con- 

 ditioned upon the length of the rays by scale relatively to the 

 length of the base line by scale. 



Not only do rays proceeding from the same object, as introduced 

 on two pictures properly placed, give by their intersection the hori- 

 zontal position by scale of the object with reference to the base, 

 but the angle subtended on any pictorial horizon by two objects, as 

 seen from the properly plotted point of view of that horizon, repre- 



PKOC. AMER. PHILOS. SOC. XXX. 138. 2 D. PRINTED MAY 27, 1892. 



