THE PHOTOGRAPHIC WORK OF GEQLOGICAL SURVEYS. 179 
It is evident, therefore, that there are magnificent records, 
which go far to show the virtues of photo-topography, and it is 
hoped that their influence will be widely felt in the proper quar- 
ters in Australia. 
INSTRUMENTS. 
It may now be expedient to consider some of the forms of 
instruments that have been made use of or recommended. 
ConTINENTAL MetHop.—PHOTO-THEODOLITES. 
This is a combination of the transit theodolite and the 
camera; there are several forms, but usually the camera rests 
on a vernier plate, and carries either on the top or on one side 
a telescope and a vertical circle. 
The “ Bridges-Lee” instrument, made by Casella, is complete, 
but its adjustments are complex, and its use requires a large 
amount of care. 
In Laussedat’s photo-theodolite, which is of Parisian manu- 
facture, one of the latest forms has the camera resting on the 
top of a transit theodolite, having the supports, which carry the 
Y bearings of the horizontal axis of the telescope, extended up- 
wards, so that the telescope may ‘pass clear under the bottom of 
the camera. 
There is another form in which the eye-piece is placed at 
the back of a rectangular camera to form a telescope in con- 
junction with the photographic objective. The camera in this 
form rests upon a horizontal vernier plate rotating around a 
vertical axis. 
The instrument used in Italy is a camera in the form of a 
square pyramid mounted on a vernier plate, which plate also 
carries a standard vertical arm, to which is fixed a telescope 
and vertical circle, &c. The instrument is levelled by three 
screws resting on the vernier plate ; two fine platinum wires are 
stretched at right angles just in front of the plate, and determine 
by their intersection the collimation axis. The camera accurately 
rotates round the vertical axis of the instrument, and the 
proper accessories are provided for adjusting the parallelism of 
its axis of collimation with that of the telescope. A Steinheil 
aplantic lens of 240 m.m. focal length, aperture of diaphragm 
2 m.m., is used, and the size of the plate is 18 x 24 centimetres. 
This photo-theodolite has been improved by Paganini. In 
the new form the telescope is omitted, the camera itself being 
made t» act as the observing telescope, being centrally mounted, 
and rotating in all respects like the telescope of a transit 
theodolite, and having the photographic lens as the objective, 
and the eye-piece mounted on a metallic plate, which replaces 
the ground glass of the camera when visual observations are 
to be made. 
Some of these photo-theodolites are very complicated and 
delicate instruments, and are apparently not very suitable for 
M 2 
