optical 'virtual objects' have depth

[P3D Peter Abrahams <telscope@xxxxxxxxxx>]

I don't normally inflict truly hare-brained ideas on the public, but I have
puzzled this one out long enough that it has become very interesting to me:
Any lens forms an image in space, not on a flat surface (although depth of
focus of the lens might be large enough to image an object with depth onto a
flat surface.)
Take a cube made of straws, and image the front surface onto a screen with a
lens.  Then move the screen back, and the back surface of the cube will be
in sharp focus.  
The lens forms a 3-d image of the object, in air.  In what other mediums
would it form a similar image...water, glass, lucite.   
What are the possibilities of being able to see that image in that medium?  
Maximize contrast, using white straws and black surround.
I have tried very dilute milk in water, but as expected, the diffusing
action of the milk overwhelmed any image.
Very fine aluminum particles in oil, sold as a science demonstration, failed
similarly.
I believe that smoke in air, flour in gelatin, etc. would also fail.
One problem is, the cube is imaged throughout the medium, it is just out of
focus except at an area corresponding to the 'virtual cube'. 
Another problem is that any medium that could reflect light to my eye would
also scatter light, and just glow uniformly.
If some phosphorescent medium would only glow above some threshold, it might
only glow where light is focused.  Fluorescent medium--??
If a mass of polarized material was used as a medium, and the object emitted
polarized light oriented at right angles, could there be extinction at
defined areas?   Might interference, using crossed polarizers, be possible?
I believe that none of the above are feasible, which leaves:
If any 'virtual object', focused by a lens, is in fact fully three
dimensional, is it at all useful in that form?
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Peter Abrahams, telscope@xxxxxxxxxx
the history of the telescope, the microscope
     and the prism binocular


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