Day/night diff in 3D perception?

[P3D <PTWW@xxxxxxx>]

I was recently showing some of my Realist format slides to a person
unfamilair with stereo photography.  He was able to easily perceive
the 3D effect on almost all the slides, as well as some printed images
in a book with the aid of a hand held viewer.  However, on several
slides, he said all he could see was two separate images, despite
looking into the viewer ($3 version) for quite some time.  All of
the slides he was unable to see in 3D were taken at night.  The
subjects of the slides were lighted and adequately exposed.  There
are varying amounts of dark areas at various depths within the scenes.

Of course I've heard that some people cannot perceive the 3D effect
in stereo slides, but I had not previously heard of a day/night
disparity in 3D perceptive ability.  Is this common?  Is there
another explanation I may be overlooking?

Paul Talbot


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