Re: Lunar Illusion

My reasoning is a little convoluted, so bear with me. :)

The problem I want to tackle is exactly where in the sequence of events
the illusion actually occurs.

There are several elements in any optical illusion - the light has to
enter the eyes, which are of different sizes and different offsets.
That is why we have stereoscopic vision. :)

Once the light has entered the eye, it gets processed by the retina.
The retina has a mix of rods and cones for processing greyscales and
colour. They are not of uniform density and do not have uniform
responses - in time, frequency or magnitude - but I don't know of any
optical illusion that works at this level. Certainly the moon illusion
won't.

The optic nerve then carries the data to the brain, which then does all
kinds of image processing, not to mention relating images to memories.
There is some leakage between the senses in all people - some more than
others - but it is the image processing and the memory relationships
that are the cause of many optical illusions.

Ok, so how would we narrow down the possibilities? The obvious first
step is to capture what the eyes would see, then replay that data to
the eyes at a later date.

If the illusion is in part a property of stereoscopic vision (ie: the
eyes are seeing slightly different images, and the brain interprets
that as the moon being slightly larger), then replaying those different
images to each eye will always reproduce the illusion, even if the
images show the moon to be the normal size.

If the illusion is entirely a property of stereoscopic vision (ie: no
interpretation is required), then a composite stereoscopic photograph
will show a moon that is larger than it "should be" for the camera
settings.

If the photographs cannot produce the illusion, then it is really not
about what you are seeing at all, it is about what your brain is
perceiving. A "real researcher" would then carry out all kinds of tests
to see exactly what was involved in the illusion occuring, but as an
armchair "illusionologist" I will content myself with knowing if it is
a mechanical (ie: to do with the eyes) or neurological (ie: to do with
the brain) illusion.

The idea is to isolate one characteristic at a time, and see if that
characteristic can reproduce the whole of the effect. If it can, then
that is the likely cause of the illusion. If it cannot at all, then it
is unimportant.

It is very hard to test the retina or the brain, but it is extremely
easy to test the mechanics of a stereoscopic optical system, and it is
also easy to test to see if the illusion can be produced by a
reproduction of what each eye would have seen.

To go much further, you'd need to muck about with white light
holography (yes, this does exist, but it is somewhat rare), optic nerve
induction, and all kinds of other esoteric tricks. None of which are
even remotely within the reach of any but the most dedicated of
researcher. As such, it's best to ignore those tests we can't do, and
concentrate on the ones that are entirely practical.

Jeff R wrote:
> "imipak" <imipak@xxxxxxxxx> wrote in message
> news:1119662924.626668.201770@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> > The illusion of the moon being larger, when close to the horizon, is
> > well-known and well-documented. Explanations are varied and range from
> > a page or so of text to entire books. Ptolemy's explanation of the
> > illusion was that there is a frame of reference near the ground and not
> > in the sky. This explanation is generally held to be the most likely.
> >
> > The illusion does NOT occur if you close one eye, and also does NOT
> > occur when using a conventional modern camera.
> >
> > This, however, does NOT answer the question of whether the effect
> > exists when using a stereoscopic camera. These have existed since
> > Victorian times, so somebody has presumably taken a picture of this
> > kind. Does anyone know whether the illusion shows up in this case?
>
>
> Not sure I understand your reasoning here.
> (I've taken lots of stereo pairs, so I understand *them* !)
>
> The effect is a naked-eye effect.
>
> If reproduced photographically, then the "size" of the moon is dependant on
> focal length, enlargement etc etc; i.e. it is dependant on the reproduced
> image size.
>
> Are you suggesting that a stereograph will reproduce the image scale of the
> moon larger than a simple single photo? ...or that the combination of the
> two images will tend to "enlarge" them?
>
> Sorry.
> I'm not making myself clear.
>
> My question:
> How could a stereograph perpetuate this illusion?
>
> --
> Jeff R.

.

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