(239) 
each differing in density from the other by 0.15 and developed in a devel 
oped in a developer of genol (2 g), hydroquinone (5 g) at 18° for 1—V2 
minutes, after which they are evaluated by means of a densitometer 
equipped with photoelectric cells placed in opposition. 
(2) Contact prints on paper from a suitable negative selected from a series of 
10 test negatives representing an “experimental terrain” (this is the name 
given to reproductions of the same piece of marquetry taken on different 
types of films and differently developed in such a way that the test nega 
tives show a contrast-difference in their extreme densities — in arithmet 
ical progression in steps of 0.2 — simulating a piece of country (fields, 
woods, open grassland) seen from an aircraft). A set of ten test negatives 
has been sent to the departments of the I.G.N. which are the greatest 
users of photographic papers. "When the paper to be tested and the test 
negative are correctly matched, the print taken should have approximately 
the same appearance, whatever the test negative and paper employed. 
The conclusions to be drawn from these tests are as follows: 
(a) Truly soft papers are extremely rare; 
(b) Few papers are extremely hard; 
(c) Fairly large fluctuation in the speed of papers bearing the same name and 
the same number and made by the same firm (the ratio may be as much 
as 2.5) and a comparatively small variation, but having more serious con 
sequences, in their gradation (differences of 0.2 maximum); 
(d) Insufficiency of the 4 or 5 graduations produced by the same manufacturer 
in papers of the same kind (bromide, gaslight, etc.) to give the best print 
for every kind of negative; 
(e) Desirability of standardizing papers, which is not done in many countries; 
(f) Well-marked graduations based on the total gradation. The best definition 
appears to be the difference between the abscissae of the points S and Q 
on the characteristic curve, S being the point where the tangent at S has a 
slope 0.2 times that of the second tangent drawn from S to the curve, Q 
being a point in the area of overexposure where the slope of the tangent 
is half that of the second tangent drawn through S; 
(g) Advantage of the “experimental terrain” method for gauging the aesthetic 
qualities of papers and their suitability for processing and use; 
(h) Advantage of aerial photographs showing a contrast of roughly 0.8 (no 
zone too dense for plotting and a large choice of papers for easy printing; 
(i) Interesting properties of chlorobromide papers. 
Tests of the resolving power of papers have been found useless, since the 
papers were quite “sharp” enough for the contact printing of aerial photo 
graphs. 
The I.G.N. has also carried out numerous tests to determine the distortion 
of positive emulsions laid on ordinary paper, high wet-strength paper and alu 
minium reinforced paper. A systematic study has been made of the various 
methods of drying. Glazing is not to be recommended if distortions are to be 
small and as isotropic as possible. Unglazed shiny papers are used for stereo 
preparation and ground completion. High wetstrength papers are of the high 
est value for mosaics, “photoplans” (mosaics of rectified photographs), and the 
rectification of panoramic photos, etc.