112 QUELQUES QUESTIONS D’OPTIQUE PHYSIOLOGIQUE, DISCUSSION
with two different types of test methods. One of
them is with a full contrast target viewed through
a pocket stereoscope. Particularly for those who
are not accustomed to a stereoscope it induces
a certain strain, either physical or psychological.
The other test is with a polaroid Vectograph
test target, the so-called Wirt test, which does
not cause much strain on the tested person. It
separates the two pictures with polaroid light.
The correlation between those two methods
is quite weak and that is the reason why I fa-
voured both, or in any case one of the test
methods, is not good; and I think none of them
is good. I think for photogrammetrical purposes,
for testing the people engaged in photogram-
metry, it is essential to have good testing meth-
ods of the stereo acuity, and I have tried to
achieve that to a certain extent in prototype
testing instruments which I described briefly in
the paper.
I may mention that my unfavourable expe-
rience with existing test methods is based on
both methods of about one thousand persons.
That is quite a lot of material, but some inter-
esting points have come out, for example about
the influence of age, the influence of sex, pro-
fession, education, etc. However, I do not intend
to go into details.
I should like to end my very short remarks
with a recommendation, perhaps I should call
it a proposal, for the resolution session: that a
working group is formed in Commission I to
study stereo acuity and to develop, or to try to
arrive at a standard method or recommended
method, for testing stereo acuity. The other part
of stereo vision is also very important, but one
has to limit it.
Mr G. G. Lorenz: I am sure all of you are
familiar with the unbalanced stereo image phe-
nomenon, so I will not go into an explanation
of it, because of shortage of time.
The paper which I have presented describes
a practical experiment in determining the
quantitative effect of measuring elevation with
a moving floating mark in a stereo plotter. In so
far as is known, no quantitative data has been
published concerning the magnitude of this ef-
fect in stereo plotter operations, although data
has been published for other types of obser-
vations with unbalanced illumination. This
experiment, which is conducted at the United
States Army Research and Development Labor-
atories at Belvoir, Virginia, used a sizeable
stereoplanograph with six inch photography
exposed at 10,000 feet altitude, and a model
scale of 1 : 13,000.
The result of the experiments can be seen on
this slide [not reproduced here]. Effectively, we
vary the illumination. This indicates the unba-
lance of illumination, and this the speed of the
floating mark: in one case it is 1.8 millimetres
per second and in the other it is 4.8 millimetres
per second. As an example of the magnitude of
this effect, if the white unbalanced ratio is 0.6
and we have a speed of 4.8 millimetres per
second at a scale of 1 : 13,000, we have an error
in elevation of about 43 feet.
Mr HoTHMER: A great variety of factors
contribute towards stereoscopic measurement.
I want to restrict myself to stressing just one,
and that is the relation *perception" versus
“conception”, which plays an important role for
an operator when taking measurements. Let me
just compare perception with stereoscopic or
intersection-registering devices for parallaxes.
Perception and conception are knowledge a
priori. Both of them contribute towards meas-
urements, and the relation between both has
some importance for an operator. He might
work with 80% perception so he can rely for
80% on his stereoscopic acuity, whereas 20%
for his measurements comes from his knowledge
of terrain features.
Mr Fagerholm’s proposal is very good, and
in setting up new tests we should consider that
the relation perception versus conception is
much better than has been done so far.
Monsieur R. DANIEL: Je voudrais dire que
d’après l’expérience que nous avons, quelque
perfectionnés que soient les tests d’acuité sté-
réoscopique qu’on pourrait imaginer, je crois
que la valeur intrinsique, la valeur pratique de
ces tests restera limitée. Je crois que ces tests
constituent plus une façon d’éliminer des gens
inaptes que d'accepter des gens aptes, et l'ex-
périence que nous avons, je le répéte, prouve
que bien souvent des gens qui ont subi avec suc-
ces les tests d'acuité stéréoscopique actuels,
peut-être imparfaits, se revèlent par la suite in-
capables d’être opérateurs sur un appareil de
restitution parce qu’il intervient d’autres
éléments qu’il est difficile de prendre en ligne
de compte dans le test, telle que par exemple
l’habilité dans le tracé des courbes de niveau.
Mr C. A. J. VON FRIJTAG DRABBE: I have
listening all the morning, one of the principal
facts in stereoscopy when viewing through à
stereoscope is that they transmit mentally one of
the images before the other eye. You can see
that immediately if you put on one class of
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