5 
Other outstanding characteristics of mechanical projection plotters originate from 
the simplicity of their optical systems of observation. So, for instance, the optical system 
includes only five reflexions in both the Stereocartograph Mod. IV and the Stereosimplex 
mod. III, while in the Stereosimplex mod. II (second-order plotter) the reflexions are cut 
down to three. 
On the other hand, observation of the photograph in a direction normal to it under 
the same apparent magnification of all its parts, besides helping image brightness, pro- 
vides an easy stereoscopie vision, with no need for any additional optical device. Also, the 
partial optical model appears in its normal angular position all over the stereogram, thus 
facilitating the tracing of countour lines. 
It can be finally said that mechanical projection plotters easily accommodate photo- 
graphs taken by any type of camera, within rather large limits of focal length, field and 
picture size. 
In Santoni's plotting instruments manufactured by Officine Galileo, Florence, provi- 
sions are especially made to allow passing with continuity from the plotting of nadiral 
photographs to that of obliques and even, in some models at least, of ground exposures. 
The use of obliques is to be preferred to that of nadiral photographs for middle and 
small scale mapping of large areas, as being more rapid and economic.