COAST. I ARTILLERY JOURNAL, - Air Defense Artillery

442 THE COAST ARTILLERY JOURNAL
part by preliminary measurement in preparation for taking impressions
(setting up of the instrument and fixing the angles a, B, g, as well as in
measuring the distance B), but in part also by measuring the film and
in putting the apparatus away.
Sources of error may be classified in seven different groups having
greater or lesser influence on the results of measurements. They
are grouped briefly, in the following notes, according to the degree of
their effects and are later discussed more in detail according to their
importance in the handling of the apparatus.
Error
No.1:
No.2:
No.3:
No.4:
No.5:
No.6:
No.7:
Kind
Errors in rotating the objective ..••
Dislocation of the film layer •....
Errors in measuring elevation of the gun
Errors of the angles ex. and g, and of the distance B
Errors of measure in setting up camera
Errors in measuring time
Errors in measuring film
Per Cent
0.00-0.00
0.02-0.05
0.02-0.06
0.01-0.03
0.01-0.03
0.10-0.15
0.10-0.20
Sum of aU errors 0.26-0.55
Possible maximum of all errors
Possible minimum of all errors
Probable mean of errors
0.8%
0.2%
'0.4%
a. Errors in Rotation of Objective. These errors, which should
receive attention in all normal photogrammetry, because one determines
the angles for forward indentation from the distance measure of the
negative, in combination with the focal length of the object gla'Ss,do
not play any role in the photographic method above described, since
one determines the path of the projectile by means of the constants of
the instruments and not from the film. The sighting angle of the objective
is measured in the construction of the camera and is transferred
to the film by fixed marks, in the shape of parallel black lines. In
consequence there is no need of a calculation, according to photometric
formulas, for ascertaining the length of the measured trajectory.
It is therefore a matter of indifference what kind of objective one
uses, pro...-idedit has a large sighting angle with adequate strength of
light. The objective types of Zeiss (Tessar 1:2.7, with 47° sight anglel
and Goertz (Dogmar 1: -1.5,with 56 0 sight field.l are, at this time, best
adapted. The objecti...-esa...-ailabletoday with a relati,-e opening of
I: 1.8 do not come into consideration for these instruments since the
greater light-effect is obtained at the expense of the sighting field .and
an objectiw of that kind has no ad,-antages compared with the types
above mentioned.