126 
HINTS TO TRAVELLERS. 
ing automatically on tlie negative (at the same time that the view is 
exposed):— 
1. Trace of the principal vertical plane. 
2. Trace of the horizon-plane. 
3. The principal point of the perspective (at the intersection of 
1 and 2). 
4. The orientation of the view. 
5. A scale of horizontal angular distances for all parts of the picture. 
6. Memoranda concerning station number, serial number of picture, 
magnetic variation, barometric pressure or altitude of station, date, time, 
alignment of principal plane, etc. 
These memoranda are first written on slips of celluloid, and inserted in 
place in the camera, where they print as shadowgraphs on the negative at 
the same time as everything else. 
The internal mechanism is very accurately adjusted in relation to the 
lens at the time the instrument is constructed, and it is operated by a 
rack and pinion which carries the whole mechanism on rails either 
forward in the box, where it is automatically clamped at all ordinary 
times when not in use, or back against a photographically sensitive plate 
when the compass is automatically released and everything in accurate 
position for exposure. An optical colour screen is fitted in front of the 
lens to diminish the obscuring effect of the blue haze of distant views. 
The whole apparatus is so constructed that when it has been accurately 
levelled by the levelling screws and levels the principal optic axis of the 
photographic lens must be truly horizontal and the back frame against 
which the dry plate will be pressed will be truly vertical and at right 
angles to the principal axis. The box of the camera is best made of 
cast aluminium alloy, and revolves on a vertical axis. 
For the rest, it is not essential for photographic survey work that the 
camera should be wedded to a theodolite, though in many ways it is 
convenient that it should be. The instrument shown in the illustration 
(p. 125) has a divided horizontal limb below the camera, and carries a 
telescope on the top with a divided vertical arc for reading elevations; 
and there are verniers, clamps, tangent screws and microscopes, which 
need no special descriptive notice in this place. The particular in¬ 
strument here illustrated was made by Casella, who charges £45 for 
instruments of this type. Other instruments much more complete and 
