P3D Re: transposing stereoscope, was uncut roll (pre)viewer

[abram klooswyk <abram.klooswyk@xxxxxx>]

After a question by Peter Davis on the 17th of May 
> (...) somebody makes a viewer that can be used with uncut 
>Realist film.  (....)
within two days reactions came from Mark Kernes, John Baird,
Norm Lehfeldt, Bob Howard, Steve Berezin, Grant C. Campos,
Greg Wageman, George A. Themelis and Mike Watters.
(17 - 19 May 1999, P3d Digest 3322, 3323, 3324, 3325).
Then it was over. Remarkable, such a flashing wave, I feel
almost embarrassed to comment no less than *four weeks* later...

First, in my opinion the device should be called "transposing 
stereoscope for film strips". 

Mike Watters described 
>(...) a little gismo made from a pair of pentaprisms 
(...)
>The image inversion is all done in the prisms (...)

I remember that at the first ISU Congress (Wageningen, the
Netherlands, 1975) in the technical working party there was
some talking on transposing stereoscopes. There was a general
discussion on the design of an ideal stereosystem, where
such a viewer would fit in as an alternative (without excluding
normal mechanical transposing after cutting, on the contrary, 
one of the first aims was to mount regular transposed slides 
with automatic registration).

J.B. Clifton, an American engineer, proposed the pentaprism
solution for the transposing stereoscope, with the addition that, 
in order to get an orthostereoscopic view, a special type of 
glass with high refractive index was required. 
Later he indeed made such a stereoscope, with the special prisms, 
and the results were impressive.  

This device was made for Belplasca 4-image strips (or similar
strips from other 7P camera's). You can cut film from those 
camera's in strips of only two interleaved stereopairs,
without separating any pair, due to the progression which is
symbolized by: 1212 3434 5656.... 
(Film strips from Stereo Realist or other 5P camera's cannot 
been cut without separating one pair of images, due to the 
Colardeau progression (1-21324354657...) so *all* pairs are 
attached to each other, when you don't want to sacrifice images)

A few people in the Netherlands, who hated mounting and (nearly)
always used projection, mounted such 4-image strips in long
glass mounts where the 28 perforation strips fitted in.
(It became known as the "Dutch quad", after the 4 images, but 
after all I believe that it could also indicate the four man who 
actually used this system, for I don't believe it ever were more:-).
I believe they did have transposing stereoscopes too).

When using the pentaprisms in a fixed frame with a flat Realist 
film strip, the separation of the optical axes should be set to the
Realist camera lens separation, 70 mm. This could be a problem for 
people with a small interpupillary distance (say 55 mm or less).

Abram Klooswyk


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