Re: Star D & lenses

[bercov@xxxxxxxxxx (John Bercovitz)]

A complete-lens Holmes would strain the eyes because the eyes would
have to diverge out to see the images which are spaced wider than the
eyes.  The eyes _really_ don't like to diverge very much.  The split 
lenses actually have their optical centers spaced at the infinity
spacing of the images.  The parallel rays coming out of the lenses make
the eye look straight ahead.  Here's how it looks with the appropriate
parts of the lenses cut away:

              eye                  eye
             
              |||                  |||
              |||                  |||
             ____                  ____
            |___/                  \___|  <- that's part of a lens  8-)
           / /                        \ \
          //                            \\
         /                                \
  ------.----------------------------------.------

        |<-------infinity spacing--------->|


The eye and lens don't know part of the lens is cut away.  8-) So the object 
point is still in the same direction, straight ahead.

> I had always assumed that
> the way magnifiers work is that your eyes focus on a virtual image, but I
> guess I can see how it might be a strain to do that for a long time if the
> virtual image is only, say, 6" from your eye.

It is a virtual image, as you say, but it's at infinity.  There's an 
interesting story  that goes with this.  Magnifier "power" is nominally
250 mm divided by focal length because 250 is how far you would put your
eye if you wanted to look at a small object, but if you focus the magnifier 
to infinity you can be its focal length from the object.  _However_, if you
focus so that the virtual image is 250 mm from the eye, then the magnification
calculates out to 250 mm divided by focal length _plus_ one.  So a 4X magnifier
becomes a 5X magnifier.  Actually what we're talking about here is a loupe.
A loupe goes right next to the eye and a magnifier goes somewhere close to the
object.  But I digress.  (Not that that is so unusual.)

John B

And as long as I'm digressing, let me reiterate about the experiment:
It is very striking and convincing.  Take a pocket magnifier and put
it to your eye.  Now lower the eye/magnifier assembly 8-) until a dot
on a piece of paper just comes into focus and don't get any closer.
The dot is now at infinity.  Then try these two things:  1) Holding the
eye and dot fixed to each other, shift the lens laterally.  The azimuth to 
the dot will change (but the dot will stay at infinity if the lens is flat 
field as it should be).  2) Now hold the lens fixed relative to the dot and
instead shift the eye laterally.  The azimuth to the dot will _not_ change.
Cool, huh?


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