Dark Ground / Spotted Lens

[MichaelG.]

Here’s an interesting little booklet, courtesy of the Biodiversity Heritage Library and the University of Leeds Library.
https://www.biodiversitylibrary.org/ite ... 1/mode/1up

ON AN EASY METHOD of VIEWING CERTAIN OF THE DIATOMACEAE.

MichaelG.

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Edit: corrected the library name

[Hobbyst46]

Thanks, Michael, for this fine combination of science and sweet prose !
Can we estimate its year of publication ?

How do we translate the article's terminology to modern terms ? like, what is today's equivalent of a 1/8 lens, who sells "spotted" lenses or objectives, etc ?

[Phill Brown]

I'll kick off with 1860's.
The author was 1816-1876?
I prefer dark field for diatoms, amazing how it was possible to resolve the pores of a diatom with the optics available.
I have a watchmakers eyeglass in a horn surround that gives an excellent image.
Likely to be far from new.

[BramHuntingNematodes]

Thanks, Michael, for this fine combination of science and sweet prose !
Can we estimate its year of publication ?

How do we translate the article's terminology to modern terms ? like, what is today's equivalent of a 1/8 lens, who sells "spotted" lenses or objectives, etc ?

An 1/8 lens has a magnification of 80x and it appears to have an NA of approximately 1.3 (1.56Xsin63 deg)

[MichaelG.]

As I understand it … the Author’s “spotted” condenser lens was simply a hemisphere with a circular patch-stop.

Thus:
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910E4A6D-96F4-4A47-A87E-75F92594A48C.jpeg (105.53 KiB) Viewed 3534 times

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MichaelG.
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P.S. 1/8 lens has a focal length of one eighth of an inch

[MichaelG.]

Looks like the date was 1856 : https://journals.biologists.com/jcs/art ... m=fulltext

MichaelG.

[Hobbyst46]

Looks like the date was 1856 : https://journals.biologists.com/jcs/art ... m=fulltext

MichaelG.

Thanks.
So the spotted lens is a simple darkfield condenser ? and, when combined with the high NA objective, created oblique illumination ?

[MichaelG.]

So the spotted lens is a simple darkfield condenser ?

Exactly that

MichaelG.

[MichaelG.]

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Here is what Conrad Beck was offering in 1921
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C8D17C44-3DFE-4EBD-BFC8-625FED02B146.jpeg (127.67 KiB) Viewed 3509 times

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MichaelG.