On a Stereoscojnc Mctliod of Pliotographic Surveying. 31 



The camera adjustments are : (1) Plane of reseait to he vertical. 



(2) Horizon line of resean to he horizontal. These adjustments are 

 made with the aid of a level, fitted with a Bohnenberger eyepiece. 



The auxiliary level having been placed directly in front of the 

 camera, and its line of collimation made horizontal, the vertical axis 

 of the camera is set vertical by reference to the level of the vertical 

 circle. Then (1) is effected by turning the camera in altitude with the 

 foot-screws, and in azimuth, until the cross-wires of the level coincide 

 with their image reflected from the silvered back surface of the 

 reseau, when the bubble of the longitudinal level on the camera is 

 adjusted to the centre of its run. Eeplace the front slide and lens 

 and set again the vertical axis vertical. (2) is now effected by 

 making the ends of the horizon line of the rescaic coincide with the 

 cross-wires of the level in two positions, using for the purpose the 

 side capstan-headed screw in the base. The transverse level on the 

 camera is then adjusted, and the longitudinal level made perpen- 

 dicular to the vertical axis l3y means of the front capstan-headed 

 screw under the camera. 



The theodolite adjustments, effected by ordinary methods, are : 



(3) Horizontal axis made jjerpendicular to vertical axis. (4) For 

 collimation. (5) Horizontal axis made ijarcdlel with optical axis of 

 camera. An approximate adjustment of (5) is sufficient. 



Instrumental Constants. — These are : (1) The zero of the front 

 scale ; (2) the zero of the reseau and co-ordinates of the E-points, 

 and (3) the focal length. They may be determined in the usual 

 manner, but it is convenient to first make the centre E-point 

 coincide wdth the zero of the reseau co-ordinates, by collimating 

 directly upon the reseau plate when adjusting the camera with 

 the help of an auxiliary level, as already explained. In that 

 case, the lens requires to be adjustable horizontally as well as 

 vertically. 



The focal length / is found from the measurement of exposed 

 plates containing the images of well-defined points of which the 

 angular distances are known. Call a the angle between two points 

 of which the horizontal co-ordinates are a and h. Then — 



'^~2tana^V 4 tan-' a ''^• 



Measurement of the Plates. — It is unnecessary in a preliminary 

 note sach as this is, to enter into the construction of the measuring 

 apparatus in much detail, as a description of actual instruments with 

 examples of their use may fitly be given in a subsequent paper. A 

 suitable machine would generally resemble those w^hich have been 



