MiP ITAODE OR PILO LO GIAPLLMC. S ORVLLYAING. SU 
It would take up too much space to describe in detail our 
methods of photographic surveying, but a few hints may be sug- 
gested to those unfamiliar with the subject and whose explora- 
tions and scientific inquiries afford an opportunity for gathering 
valuable data in the field by means of the camera, which may 
be subsequently worked out. The first requisite is the utilization 
of a constant (the principal) focal length for all photographs 
taken. This necessitates a rigid camera box with no focus 
adjusting arrangement (bellows). The lens should be a good 
one, giving a flat image and having an angle of about 60°. To 
give sharpness to distant points an orange glass screen is always 
screwed to the front of the lens. This, of course, increases the 
time of exposure. In the center of each side of the box and in 
front of the plate is fixed a brass point or comb each of which 
shows on every photograph and together they serve to orient 
the photographs. The camera must always be horizontal, to 
insure which a small level is placed on top; preferably one com- 
posed of two in the form of a T. It is convenient to have a foot 
for the camera so that it can be placed and leveled on the tripod 
of the transit or theodolite necessary for making the skeleton 
triangulation, to which the photographs are referred. For good 
work glass plates are essential. They should be orthochromatic. 
A convenient size for transport is 434 by 6% inches. In pano- 
tamic views, adjoining photographs should have at least one 
well-recognizable point in common, 7. é., the photographs should 
overlap. 
In applying the camera to the study of the motion of gla- 
ciers, it is difficult to give definite instructions what to do, as the 
conditions presented vary so widely. Each case will suggest 
the most advantageous course to pursue; what length of base to 
measure ; how many pickets to plants across a glacier (it is well 
to distinguish the pickets by having alternate ones with a 
‘“‘cross-head”’ T), etc. The photographs want to be taken from 
the stations, and at intervals of time of a week or weeks, or 
months, dependent upon the conditions and thoroughness of 
investigation required. In general we may say that the same 
