8 
The test measurements performed are too few in number to draw definite conclusions concerning 
thickness variations of aerial films. However it is evident that such variations contribute to the errors 
of orthogonally observed image coordinates and that these errors will increase with radial distance from 
the principal point. Specifications for allowable variations of thickness are desirable and should be drawn 
up giving due consideration to other sources of errors. 
The measurements and computations were made by Mr. B. Gustafsson. The films were provided by 
the Geographical Survey Office of Sweden through Mr. E. Welander, Tekn. lie. 
• 
1.33 Localization of the Silver Grains in the Emulsion 
The purpose of the investigation was to examine where the exposed and developed grains of silver 
are located in the photographic emulsion and, further whether this location changes with varying 
exposure—and developing time. Such variations will introduce errors if the image coordinates are 
measured orthogonally and will also contribute to the weight variation of image coordinates. 
A high contrast test target with black and white fields was photographed with panchromatic film 
using varying exposure times ( x / 2 ,1, 2, 4, 8, 16, and 32 sec.). Three series were exposed and the developing 
time was then varied (6, 12, and 18 min.). One series was further exposed with a yellow filter and given 
normal development. Glass plates were used. 
The emulsion layers from the center of every picture were then loosened from the glass plate and cut 
perpendicularly to the emulsion surface in a microtome in 5 /um thick slices. Then slides were bedded as 
preparatory. 
The number of developed grains of silver were then counted in a microscope for the lighter and darker 
parts of the negative. The contrast was used as an extra exposure variation. 
Ten repeated measurements were made on every preparate under about 1 500 times magnification. 
As a hypothesis the center of gravity of the silver grains was then considered as the point which represents 
the image coordinate. 
In order to eliminate the effect of varying thickness of the slices and varying thickness of the emulsion 
all observations were reduced to percent figures and to the same emulsion thickness. 
The precision of the measurements was calculated as a standard deviation in one observation of 0.5 /im. 
The following calculations were intended to express the point of gravity as a function of the exposure 
and the developing time. The following function was finally developed from the total material. 
/= x x -f- t[X 2 -f- t e x 3 -J- i 2 e # 4 where 
f — distance in microns from the emulsion surface to the point of gravity 
x x v 4 — constants 
t[ = developing time (if = 1, 2, 3) 
i e : logarithm of exposure time (i e = 1 13) 
38 observations of the point of gravity were obtained. A least squares adjustment of the observations 
to the function above gave the following result 
f= 8.026 - 0.155 i f — 0.441 t e + 0.032 t\ 
Standard error of unit weight s 0 = 0.58 um 
Confidence limits on the 5 % level 
0.85 s 0 < s 0 < 1.4 s 0