P3D Re: Hugo de Wijs VM viewer, Re: Normal interocular spacing?

[abram klooswyk <abram.klooswyk@xxxxxx>]

When [Sir] David Brewster first published his stereoscope (1849,
1851) he wrote: "The semilenses should be placed in a frame, so 
that their distance may be adjusted to different eyes". 

In 1867 the famous German physiologist Hermann [von] Helmholtz
(the "von" is the German equivalent of a knighthood, so it is
not in his early writings) published his modification of Brewster's
stereoscope. I translate (:-)) a part of his German text:

"Because the distance of corresponding points in the usual
photographic stereoscope images not always is the same as that
of the eyes (...) should one, in order to achieve the most
natural projection of the objects, have the possibility to
adapt the instrument to every image" (... das Instrument jedem
Bilde adaptiren koennen).
Helmholtz also advocated looking with the visual axes parallel
when looking at landscapes pictures, as Claudet had done before.

A scheme of a stereoscope (seen from above), two corresponding
Far points F on the picture, which is supposed in the focal plane
of the lenses (Sorry, I could find only these rough ones :-)),
so there will come only parallel rays out off the lenses, from 
each point of the picture. Lens separation is made equal to stereo
picture far point separation.
Therefore three persons with Large, Medium and Small 
interpupillary distance all can look with parallel eyes at far 
points in the picture. Only Mr Medium can look though the lens 
centers, the other two use the lenses off center.
         
              .-.                    
            .'   '. --------------------------------L        
|          /       \                 
|         |         |
|F        |         | ----------------M
|          \        /                
|           '     .                 
             \   /  --------S
               '.

              .-.                    
            .'   '. --------S        
|          /       \                 
|         |         |
|F        |         | ----------------M
|          \        /                
|           '     .                 
             \   /  --------------------------------L
               '.
                                                (c) Closearts

This scheme is the viewing method recommended by Helmholtz.

The essential difference between Brewster and Helmholtz is the
adjusting of the lens separation to the eyes OR to the pictures.
When Mrs Small would adjust the above stereoscope "to her eyes", 
to look through the centers of the lenses, she would have to 
_diverge_ (look wall-eyed) to see the far points. 
Adjusting lens separation to the stereo picture's infinity or
far point separation will avoid this. 
(The picture should be kept in the focal plane, for people with 
normal - or corrected to normal - eyes, this can be done by moving
the lenses as close as possible to the pictures, while keeping 
vision sharp)

George Themelis wrote (PHOTO-3D Digest 3220, Sat, 27 Feb 1999):
>As far as I know, this is the only VM viewer with adjustable 
>interocular.  This feature, coupled with the large lenses, 
>will make viewing a very pleasant experience for everyone, 
>no matter how small their interocular spacing is (people 
>with narrow interocular spacing have reported difficulties 
>in using VM viewers.) 

The View-Master system is probably one of the best in keeping
far point separation at a standard value. Therefore VM viewers
_don't need_ an adjustable interocular! 
Only large lenses are needed to accommodate all interpupillary 
distances.

>You might get this message more than once since it is also 
>    posted in the following lists:  RBT-other, View-Master, 
>    3D-Viewers, DeWijsViewers, FansOfHugoDeWijs and the
>    3Dmillioners list! :-)

George, as moderator of the FansOfHugoDeWijs list I must tell
you that you will be out, if you spread any further confusion 
on his viewers. 

(to be continued)

Abram Klooswyk


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End of PHOTO-3D Digest 3222
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