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6 
may cause a variation in the distorsion curve of certain len- 
ses; such variation is different inside of depth of field. 
This problem has been examined in the above mentioned publica 
tion (2), and we think it can be overcome by determining di- 
storsion over three levels (the outside end, the middle and 
the inside end of depth of field). Distorsion adjustment wi- 
thin these levels is calculated on the spot using a linear in 
terpolation based on a computing program that we have studied 
for the purpose, 
Experiments on the matter are still under way. 
Let’s point out that the reliability of the distorsion 
curves determined according to the procedure suggested by us 
is strictly dependent on the precision we can obtain in the the 
oretical determination of the plate-coordinates of a given num 
ber of points, following our computing program, This problem 
has been studied in theory and the results are not still pub- 
blicated. 
Bear in mind that further errors may take place if plates 
are not perfectly flat, or gelatine has shrinkaged, or tempera 
ture has changed greatly. 
The signalization of the points to be read out is of para 
mount importance in the analytical method, lt is obvious that 
this signalization, besides being troublesome, is also restri 
ctive, We solved this problem by materializing a number of Li 
nes along certain directions and by marking the necessary po- 
ints with self-adhesive marks. 
This operation is rather quick, allows concentration on 
the sectors of major interest and does not damage the object 
under survey; marks have to be studied according to the scale 
of photograms and to the required enlargement; possible discre 
pancies depending on reading out being analogical rather than 
analytical, shall also be taken into account. In addition to 
the marks located on the lines we may have other points posi-