APPLIED STEREOSCOPY 205adjacent parallactic differences. The process worked all right, but
the picture was of course crossed by vertical black lines which
were visible and marred the picture. Later the use of lenticular
elements embossed upon a plastic base was suggested by both
German and French experimenters, and in 1925 such grids were
successfully used in France. Among the details introduced were
specially shaped diaphragm apertures and a moving diaphragm
which caused a slit aperture to move across the base aperture. And
these things are now almost a quarter century old, and of course
the essential characteristics have long since become public prop-
erty.
Briefly, the lenticular grid makes the stereoscopic depth appar-
ent throughout a greater viewing angle than the line grid, but
nevertheless the junction of the elements produces a line effect.
The only way to overcome the line effect is to make the grid so
fine that the lines are invisible as such. With a screen as fine as
this, registration becomes difficult and the images are processed
by reversal, a complication which removes any process from full
practicality, even though it is widely used. No process can hope
to be truly successful until there is complete freedom of repro-
duction, preferably by the usual negative-positive process.
The equipment necessary for making the bar-screen stereo-
grams consists of a camera; a lens whose diameter permits the use
of two apertures with a center-to-center separation of 65mm;
and the essential grid screens which may be either integral with
the sensitive material, or separate. (Eventually the large-diame-
ter lens may not be necessary as the writer has been informed that
applications for patent have been made covering a system which
permits the use of any normal small camera lens, and which also
maintains the vital grid angle with accuracy.)
Under present day conditions, unless one is lucky in having
old equipment from which to draw parts, the initial cost will run
about 500 dollars, and of course the results are not worth it!
The writer has made a great many bar-screen stereograms using
screens of various types, both line and lenticular, and it is his
considered opinion that as far as good quality stereoscopic photog-
raphy is concerned, the process is of little interest.
On the contrary it must be said that as far as commercial appli-
cation is concerned, particularly if these pictures eventually can