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Unusual Cooke lens - Microscope?
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PostPosted: Mon Oct 31, 2011 1:16 am    Post subject: Unusual Cooke lens - Microscope? Reply with quote
Hi folks

I bought an unusual lens that I suspect may be for a microscope, the only markings on it are:

Cooke 40949 D.G.

It has a thread mount approximately twice the diameter of a normal RMS microscope thread mount and is very squat, only a third as tall as it is wide.

Looking into the lens from the front all I can see is black, looking into it from the rear, it has a silvered surface like a blob of mercury and just reflects my own face back at me.

Anyone ever seen anything like this before? Google finds nothing.


PostPosted: Mon Oct 31, 2011 1:21 am    Post subject: Reply with quote
I have no knowledge about this subject matter, I suspect however that the people that do will ask for a picture of the lens in question. Wink


PostPosted: Mon Oct 31, 2011 1:30 am    Post subject: Reply with quote
Does light pass through it?

Also yes a picture would be interesting.


PostPosted: Mon Oct 31, 2011 1:34 am    Post subject: Reply with quote
No light passes through the centre but there is a thin transparent region around the edges of the silvery reflective spot.



PostPosted: Mon Oct 31, 2011 8:07 am    Post subject: Reply with quote
It could be a Schwarzschild objective.

http://spie.org/x34462.xml


PostPosted: Mon Oct 31, 2011 9:09 am    Post subject: Reply with quote
It is far more likely to be a dark field lens.


PostPosted: Mon Oct 31, 2011 11:48 am    Post subject: Reply with quote
Certainly not a Schwarzschild reflective objective, as it is way too small - Cooke reflective objectives look like that:


I agree with sevo that it might be a Darkfield objective, as the center rays are blocked out.


PostPosted: Mon Oct 31, 2011 1:08 pm    Post subject: Reply with quote
Yes, it seems to me that the D.G. on the lens stands for 'Dark Ground' so it will indeed by a darkfield lens.

I wonder what magnification it is and if it will be of any use for macro work?


PostPosted: Mon Oct 31, 2011 4:06 pm    Post subject: Reply with quote
Works only for transmitted light using a special condensor system.


PostPosted: Mon Oct 31, 2011 4:10 pm    Post subject: Reply with quote
Ah well, it was one of a job lot of Cooke microscope lenses I bought very cheap, the others will prove useful.


PostPosted: Mon Oct 31, 2011 9:13 pm    Post subject: Reply with quote
But what does a dark field or "dark ground" lens actually do/what is its purpose? Going by the description given by the OP, I assumed it was used in a telescope to block out the suns disc so that the suns corona could be observed safely by eye but that cant be right can it? Question


PostPosted: Mon Oct 31, 2011 11:43 pm    Post subject: Reply with quote
DSG wrote:
But what does a dark field or "dark ground" lens actually do/what is its purpose? Going by the description given by the OP, I assumed it was used in a telescope to block out the suns disc so that the suns corona could be observed safely by eye but that cant be right can it? Question
http://en.wikipedia.org/wiki/Dark_field_microscopy
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