30 TicDisactions of the South African Philosoijlilcal Society. 



along a line parallel to that joining the nodal points of the two eyes, 

 and for different distances bet\Yeen the viewing lenses or the eyes, 

 since in all these cases the lines joining the two images of a point 

 remain parallel to the eyes. 



SiLvvcjjing Camera. — The essential features are a camera on a 

 theodolite base, and a telescope with its line of collimation at right 

 angles to the optical axis of the camera, so that by changing pivots 

 the orientation of the pair of plates is not affected by errors of in- 

 clination, collimation, or graduation. 



The photographic plate is pressed, during exposure, against a l^ack 

 frame in the focal plane of the camera lens by a spring contrivance 

 similar to those used in other surveying cameras, which permits the 

 shutter of the dark slide to be drawn and replaced. The reseaic is 

 hinged in front of the plate, its correct register being determined by 



Fig. 2. 



geometrical contacts. It is impressed upon the plate by exposure 

 to sky-light reflected through the camera lens, and then moved out 

 of the way for the exposure of the picture itself. A graduated front 

 slide is used to displace the horizon line by moving the lens, but in 

 normal circumstances it is set at the zero of its scale. Fig. 2 sliows 

 the general arrangement of the instrument. 



Conditions to be satisfied. — One instrumental condition, sufficiently 

 satisfied in construction, is that the front slide be parallel to tlie 

 vertical reseau lines. Any defect in this respect is eliminated Iw 

 determining the origin of the reseau co-ordinates and the focal 

 length for different readings of the front scale. 



