Re digest 130

[T3D Peter Homer <P.J.Homer@xxxxxxxxxxxxxxxxx>]

Sorry for the late reply but our network has been out of action untill
today 7/4. While it is clear that nobody has been in rapture about the
discussion on lens depolarisation the point is it can be avoided which is
probably why it has seldom been noticed. I have examined a number of lenses
many of them"identical"
between crossed polars and while I have found many that show a cross of
polarisation with "depolarisation" in the remainder of the lens others even
when Identical show it to a much lesser degree or it may be absent
altogether and retain perfect extinction . I wonder if John Bercovitz even
noticed the same affect with both lenses of his projector . This tends to
suggest that the geometrical explanation which I must confess I still dont
fully understand can only be part of the explanation if at all and that
some sort of crystalisation affect which I do understand better is also
involved. Something I have noticed since repeating the experiment is that
if the lens is rotated the cross if present usualy seperates from the axis
into two curves made up from the arms of the cross at right angles to one
another which move to the outside of the lens as it continues to rotate
then without a change in the direction of rotation of the lens they start
to come back in again to reform the cross . Then seperating again in the
opposite directions before returning again etc with the cycle repeating.
 How would the geometric explanation acount for this ? it is however
typical of the behaviour of a Biaxial crystal (one with two directions
which have no affect on polarised light) observed in rotation under the
polarising microscope. Except
that there the cross is not observed in the specimen itself but in the back
of the microscopes objective ( which itself should not and does not produce
any cross ). This can be done simply by removing the eyepiece but proper
polarising or petrographic microscopes are usualy fitted with an extra lens
called an Amici-Bertrand lens which can slide into the optical path to
produce an enlarged image of the back of the objective . It is also
neccessary for the polarised light to be convergent for this and this may
not seem like the same set up as I have used for viewing lenses. But the
purpose of the microsope is just to enlarge the image of small crystals
while the lens itself probably acts as its own converging and Amici-
Bertrand lens . I suppose I should be bothering the sci-optics newsgroup
that the article originaly came from rather than this group but I no longer
have access to Newsgroups since I found that Alt-3D had very little to do
with stereo.   P.J.Homer


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