use two « test » original negatives. In order to have in these « tests » 
the entire transparent field, they can be submitted to well known 
inversion baths. To avoid the inversion, which is preferable, 21 
the moment of the impression of the «test » there can be applied 
to the taking camera an opaque screeen (preferably metallic) imme 
diately underneath the focal plane, this screen having small holes 
situated in accordance with in the reperes. 
The plotting instrument must posses adequate attachments (screws, 
levers or similar) for the purpose of moving the photogram in the 
right plane so that one of its marckings will coincide exactly with 
a corresponding one of the « test » which is situated closer to the 
zone in which the plotting takes place. The movement of the pho 
togram, rather than manually can be achieved in an automatic and 
progressive fashion between one repere and the next, if the device 
is supplied with adequate components which for example the Ste- 
reocartograph Santoni Mod. V is provided with.