Re viewer interocular

[P3D <PgWhacker@xxxxxxx>]

>I was a little puzzled by Greg Kanes use of the term "Optical Centre"
>simply centre would seem to suffice.

     The lenses in Holmes stereoscopes are unusual in that they are 
really chips taken from the perimeter of a larger lens.  Typically the 
larger lens, the one the stereoscope lens-chips are taken from (at least 
conceptually), has a diameter of 2 inches, 4 inches or more.  For a 
Holmes stereoscope you don't need -- can't use, wouldn't want to pay for 
-- two honking 4 inch lenses.  

    But, to be able to view stereographs wider than about 2.5 inches you 
need lenses with the optical properties of the larger lens -- 
specifically you need two lenses whose centers are at least as wide as 
the infinity point separation of the stereograph.  So, conceptually at 
least, you take a large lens, trim out chips from it's perimeter, and 
mount the chips in the stereoscope.  The chips behave just like the 
larger lens would.

    Of course speaking about the "center" of such a chip tells you 
nothing about it's optical properties, thus the term "optical center,"  
which refers to the center of the original large lens.


   By the way, you _can_ make a stereoscope using exactly the chip 
removal technique.  Buy one of those plastic fersnell magnifying sheets 
at an office supply store;  cut a chip from each end of the sheet;  mount 
the chips on a cardboard frame and, Bingo, you've got a stereoscope.  Be 
warned though, the image quality leaves a lot to be desired.






Greg Kane
Denver

PgWhacker@xxxxxxx





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