Re: [Histonet] Dark ground illumination for silver

From:Philip Oshel

Hum. I wonder if this is a case of Wilde's "two people separated by a 
common language"?
By "darkground" microscope, do you mean a darkfield microscope? If 
not, then ignore what I'm writing, if yes ...
"So, what is the specificity/range of things that shine through 
darkground illumination?"
There are 2 basic answers. For opaque objects like many mineral 
deposits, anything that diffracts light well enough will be bright 
against a black background. For objects that transmit light, then 
anything that has a refractive index sufficiently different from the 
surrounding material will be bright against a black background. 
"Sufficiently different" just means being different enough to scatter 
the light, so ordinary biological samples are good for darkfield 
microscopy. The same principle works for darkfield transmission 
electron microscopy.
The basic principle is a hollow cone of light, of a diameter 
exceeding the NA of the objective, is projected through the sample. 
Undeflected -- by diffraction or refraction -- light is not 
collected, and so the field is black. Diffracted or refracted light 
is directed into the objective and so imaged. This also allows 
detection (although not resolution) of sub-resolution structures, 
such as bacterial flagellae.
Darkfield microscopy was developed mostly for biological samples, I 
believe. It's great for cells, bacteria, protozoans, tiny critters in 
pond water ...
But useless for identifying minerals or metals beyond "Yep, somthing's there."
Here's my chance to plug one of my favorite books I don't see 
mentioned on Histonet: E.M. Slayter and H.S. Slayter, "Light and 
Electron Microscopy". No how-to, but excellent discussions of how and 
why microscopes do what they do.

If you meant some other kind of microscope, like one used to search 
for metal ores in black loam soil, then all that doesn't apply.


>I don't know why, but all my best stories are against myself. This 
>is the latest.
>I recently had a skin in which there was finely particulate pigment, 
>rather deep, and slightly but definitely favouring sweat gland 
>basement membranes. "Ah" thought I - "got it - silver".
>The next port of call was microbiology for a darkground microscope. 
>Delighted I was when the material shone through like the stars in 
>the desert. As readers may anticipate, that is exactly what silver 
>was supposed to do.
>I sent it to the local centre of excellence for EDAX, having already 
>spoken to the recipient and telling him that he would be receiving 
>it, but I was going to do an iron and melanin stain first.
>It got neglected, and I suddenly refound it and in a hurry sent it 
>off without doing these stain.
>Yes readers, the report came back that they had stained it for iron 
>and it was positive.
>I have implored the recipient to not tell anyone in exchange for 
>good money, but I'm sure it will be too late.
>So, what is the specificity/range of things that shine through 
>darkground illumination? I can't find the answer. From the fact that 
>this test was mentioned for silver, there seemed to me to be an 
>implication that other common things like iron would not so behave.
>Can anyone put me right?
>Dr Terry L Marshall, B.A.(Law), M.B.,Ch.B.,F.R.C.Path
>  Consultant Pathologist
>  Rotherham General Hospital
>  South Yorkshire
>  England
>Histonet mailing list

Philip Oshel
Supervisor, BBPIC microscopy facility
Department of Animal Sciences
University of Wisconsin
1675 Observatory Drive
Madison,  WI  53706
voice: (608) 263-4162
fax: (608) 262-5157 (dept. fax)

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