(238) 
The laboratory experiments were carried out by the Research and Devel 
opment Department of the I.G.N., using photographic targets set in rotation 
in such a way as to present, at the same time, variable tangential speeds. 
These targets presented a variety of contrasts and were composed of 
circles, dots or single lines, as well as such features spaced at regular intervals. 
The experiments in flight were carried out by the Groupe des Escadrilles 
Photographiques (Photographie Group) by flying over the Creil airfield on 
which various targets had been disposed, some at regular intervals, some not. 
These experiments may be considered to have shown: 
(a) Despite statements sometimes heard to the contrary, such blurs do exist 
and their actual length is only slightly less than the theoretical length (80 
or 90% of it as a rule). This small difference may be attributed to the 
threshold of sensitivity of photographic emulsions and their thresholds of 
capability of recording contrasts; 
(b) the blurring decreases, though relatively little, with the contrast; 
(c) irradiation may produce effects which are not to be confused with those 
due to image movements; 
(d) a study of this blurring has shown that the effective time of exposure is 
closer to the period during which the shutter is fully open than to the 
total period during which it is in action. 
Certain general conclusions have also been reached which may interest the 
photographic surveyor. These are concerned with: 
(a) the reasons why some people in all good faith deny the existense of these 
blurs (through having required insufficient sharpness of the prints, being 
accustomed to artistic photography with its more or less imprecise out 
lines, too seldom using an enlarging device, a magnifying glass or, better, 
a stereoscope, etc. . . ., or owing to the lower contrasts common in aerial 
photography, the uncertain outlines of cultivated areas, etc. . . ., and the 
very great difficulty of deciding at which point in the shading with which 
the blur ends the actual line is to be taken); 
(b) the disadvantages of this blurring (lack of sharpness in the pictures and 
lack of precision in setting the mark). These are only serious to some 
extent in the very large scale photographs necessary for cadastral surveys; 
(c) the remedies (rather than displacing the plate or film to compensate for 
that of the terrain — a procedure which would ill accord with the 
demands to be met by a metric surveying camera, it is better to use a heli 
copter for ultra-large-scale surveys). 
PRACTICAL METHODS OF TESTING SILVER HALIDE PAPERS 
INTENDED FOR PRINTING AERIAL PHOTOGRAPHS 
by 
J. Cruset, Ingénieur Géographe Institut Géographique National, France. 
The sensitivity test to which photographie papers used by the Institut 
Géographique National for printing aerial photographs are subjected consist of: 
(1) The printing of sensitograms with a specially designed printer through a 
photometric wedge 21 cm long by 4 cm broad, consisting of 21 equal areas