David Wolfe (davidwolfe3@earthlink.net)
Sun, 10 Oct 1999 00:21:48 -0700
On Oct 9 Liam Lawless wrote:
>I'm doing some experiments and need a close look at the silver deposits of a
>couple of silver-gelatin prints produced by "alternative" means; I expect
>that the individual grains (which I want to know about) are too small to be
>seen with an optical instrument. Anyone out there have access to the gadget
>in the subject line? Thanks, Liam
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Liam:
I certainly have access to both ordinary transmission electron microscopes,
as well as scanning EMs, and would be happy to help you out; but you need to
say much more precisely what you're after, because I suspect you may not
need an electron microscope to answer your questions or, if you do, trying
to directly image chemically developed and fixed silver grains embedded in a
silver-gelatin print--though technically possible--is not likely to be a
rewarding technique. What are you trying to determine, exactly? Give me a
little feedback, to know how best to help you; knowledge in several
disparate fields may, or may not, be relevant to your concerns.
For example, biomedical researchers have been doing electron microscopic
autoradiography for about forty years; this technique involves laying down a
true monolayer of silver halide crystals (special research emulsions with
super-fine crystals sensitive mainly to low-energy electrons instead of
photons in the visible or UV spectrum) over a very thin (~800 Angstrom
units) epoxy section containing point-sources of radioactive material (eg,
tritiated thymidine) whose radioactive decays (electrons: beta particles)
form the latent image, which is then developed (by non-compensating
developers, exploiting the fineness of the original halide crystals) and
fixed, before viewing by electron microscopy. From consideration of the 3D
relation of the radioactive point-source in the underlayer to the overlying
monolayer of silver halide particles, the physics of their electron-capture
(sensitization), and the process of development of the elemental silver
'grain'--which grain looks like a coiled wisp of malformed spagettini--the
spatial resolution of the technique can be calculated, generating
electronmicroscopic autoradiographs localizing, to that resolution (no
better than 0.1 micron in the above example), the radiation source
(tritiated thymidine in nuclear DNA) in the cell imaged. This is REALLY
looking at the the silver grains in the 'print'! Short of electron
microscopy, several light-microscopic techniques also can image single
silver grains--confocal laser microscopes or, since each silver grain is a
mirror, darkfield oblique or epi-ilumination.
But this sort of information does not translate in any direct or simple way
to multillayered silver gelatin emulsions, in terms of acutence, contrast,
tonality range, what the eye percieves, etc. Incidentally, many relevant
matters are splendidly discussed in CTEIN's "Post Exposure: Advanced
Techniques for the Photographic Printer", Focal Press, 1997. CTEIN gives his
email address: <71246.21@compuserve.com>
Tell me more!
David Wolfe
davidwolfe3@earthlink.net
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