184 THREE-DIMENSIONAL PHOTOGRAPHYObtain a long pole; a broomstick is excellent. Have someone
hold this so that it is about ten inches from your face, pointing
toward your eyes and extending away from you. Look at the end
which is near the eyes. Now look at the other end of the pole.
What happens? To use a familiar expression you “look down the
length” of the pole. That phrase, so common in our language, it-
self reveals the movement and the time interval involved. You do
not see the whole pole at one time. In fact when you look at the
near end you see two poles stretching away in divergent directions.
Then your eye travels down the pole and the two poles cross and
finally when you look at the other end they are now convergent.
Remember this is direct vision in real life!
Be honest! Had you ever before noticed that peculiar sight? Did
you know that real, direct vision works this way? Probably not and
with good reason. About the only time we look down the length
of an object is when we close one eye and sight down the length
to see if it is straight. The two eyed experiment is not a type of
vision which is commonly used.
If you make a stereogram of that same pole, you will have the
same effect, but this time remember the focus must remain upon
the screen surface and the nearer the near end of the pole is, the
greater the separation of focus and convergence demanded.
If you have acquired normal stereo skill you will have com-
pletely severed that bond and you will find the stereogram looks
just like the pole does in real life: but what of the newcomer to
stereo?
He will try to look at the near end of the pole and either the
images will spring apart and he will see dual images, or focus will
follow convergence and the whole picture will blur. He can feel
the pull of the two sets of muscles trying to act independently and
he will believe he is injuring his eyes. He will cry, “Oh! That is
intolerable. It hurts my eyes. I can’t look at it!” Of course he can-
not, he has not what may really be called a normal degree of visual
skill.
This having to “look down” a long length receding into the dis-
tance with the consciousness of dual images is sincerely thought by
most people to be wholly different from anything in ordinary life.
When to that is added the pull of a considerable degree of separa-