presents in practice well known inconveniences. In fact, having to 
have a notable thickness, be it for the necessities of optical work 
or for its stability, it would determine in the picture a notable distor 
tion which must be noted in the lens calculation. 
It is not therefore applicable to numerous existing cameras having 
excellent lenses practically free of distortions. Moreover because 
of the inevitable fall of dust particles and because of the contact 
with the film, the slide is subjected to scratches which then are con 
stantly reproduced on the picture. 
And these reference markings, no matter how they are produced 
(blackened markings metallic projections etc.) be it because of the 
rubbing of the film or the necessary cleaning of the glass, can easily 
be impaired. 
It is known that to realize the markings optically of the marginal 
reperes, already, in the past, there was placed along the edge of 
the frame small lenses of short focus (for example 2 to 5 mm.) that 
at the moment of the taking formed on the focal plane the picture, 
very reduced, of the lens diaphragm constituting in this same pic 
ture the reference mark. Sometimes, especially if the surface of a 
lens of the objective came to rest in the proximity of the diaphragm, 
in the centre of this surface, there was carved a circle or a small 
opaque disc of which the picture, notably reduced, formed itself 
on the plane of the film thus constituting a marginal mark of refe 
rence of a more definite indentification and of constant dimensions, 
independent of the opening used for the diaphragm. 
The device conceived by Santoni still relies on the use of similar 
small lenses but of greater focal length (10-15 mm.) meant to form 
in the focal plane pictures of the diaphragm in its centre for example 
on the lens plane on which there is cut a disc, a point, a small cross 
or facsimilie, or even two crossed wires. Said small lenses are pla 
ced other than marginally, even in the inside of the field. Towards 
this end they are maintained in position by mechanical organs that 
do not disturb the continuous formation of the picture. Such organs 
of support are preferably made up of steel straps (n) (figg. 1-2-3) 
stretched between the opposite sides or between the corners of a 
frame (c) suitably fitted on the photographie camera. The straps