Dark field illumination: Questions and answers

Here you can discuss different microscopic techniques and illumination methods, such as Brightfield, Darkfield, Phase Contrast, DIC, Oblique illumination, etc.
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apochronaut
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Joined: Fri May 15, 2015 12:15 am

Re: Dark field illumination: Questions and answers

#61 Post by apochronaut » Fri Apr 24, 2020 1:17 am

I'm not exactly sure of the era of the objectives, probably from about the first W.W. to the early 50's but the design is a split shroud, with a lens section below a knurled ring and a blank section with rear baffle only above it. You unthread the baffle section and replace it with the iris as an adjustable baffle.
The old ones were brass above and likely palladium plate below. In the 30's they changed to a shinier metal plate, for both sections. Maybe chrome but possibly some other non reactive metal.
I'm a bit busy for a few days but by Sunday I can probably take a couple of pictures of one. I may have both types. Send me a p.m. early in the week and jog my memory, if I don't get it done. Planting, these days.

apochronaut
Posts: 4115
Joined: Fri May 15, 2015 12:15 am

Re: Dark field illumination: Questions and answers

#62 Post by apochronaut » Fri Apr 24, 2020 1:18 am

The iris works on any of the higher N.A. objectives above anout 60X.

Bryan
Posts: 93
Joined: Mon Sep 17, 2018 11:31 pm

Re: Dark field illumination: Questions and answers

#63 Post by Bryan » Fri Apr 24, 2020 4:47 am

apochronaut wrote:
Fri Apr 24, 2020 1:17 am
I'm not exactly sure of the era of the objectives, probably from about the first W.W. to the early 50's but the design is a split shroud, with a lens section below a knurled ring and a blank section with rear baffle only above it. You unthread the baffle section and replace it with the iris as an adjustable baffle.
The old ones were brass above and likely palladium plate below. In the 30's they changed to a shinier metal plate, for both sections. Maybe chrome but possibly some other non reactive metal.
I'm a bit busy for a few days but by Sunday I can probably take a couple of pictures of one. I may have both types. Send me a p.m. early in the week and jog my memory, if I don't get it done. Planting, these days.
Thanks for that, I found some examples what you’re talking about online.

apochronaut
Posts: 4115
Joined: Fri May 15, 2015 12:15 am

Re: Dark field illumination: Questions and answers

#64 Post by apochronaut » Tue Apr 28, 2020 1:37 pm

Here are some photos, as well.
Attachments
upper section of a 1/12" 1.30 objective and the alternative iris section
upper section of a 1/12" 1.30 objective and the alternative iris section
DSC03197 (1280x719).jpg (126.17 KiB) Viewed 1905 times
rear diaphragms of a 1/12" upper objective section left and the iris section on the right. They date from about the same time.
rear diaphragms of a 1/12" upper objective section left and the iris section on the right. They date from about the same time.
DSC03198 (1280x719).jpg (146.18 KiB) Viewed 1905 times
iris section next to a contemporary 1/12 " 1.30 N.A. objective
iris section next to a contemporary 1/12 " 1.30 N.A. objective
DSC03196 (1280x719).jpg (126.53 KiB) Viewed 1905 times
iris section fitted to a very old 7 objective, probably about 15 to 20 years prior.
iris section fitted to a very old 7 objective, probably about 15 to 20 years prior.
DSC03199 (719x1280).jpg (135.29 KiB) Viewed 1905 times
iris section fitted to a more modern 62X objective marked 7 but also 62X. it was made about 20 years later. the N.A. is not marked but it performs as about a .85, probably a little too high for the D.F. condenser.
iris section fitted to a more modern 62X objective marked 7 but also 62X. it was made about 20 years later. the N.A. is not marked but it performs as about a .85, probably a little too high for the D.F. condenser.
DSC03201 (719x1280).jpg (108.59 KiB) Viewed 1905 times

Bryan
Posts: 93
Joined: Mon Sep 17, 2018 11:31 pm

Re: Dark field illumination: Questions and answers

#65 Post by Bryan » Wed Apr 29, 2020 3:54 am

apochronaut wrote:
Tue Apr 28, 2020 1:37 pm
Here are some photos, as well.
Thanks for getting back to me on that, I’m watching a few on the evil auction site.

apochronaut
Posts: 4115
Joined: Fri May 15, 2015 12:15 am

Re: Dark field illumination: Questions and answers

#66 Post by apochronaut » Wed Apr 29, 2020 11:16 am

There are usually subtle changes in the objectives made between about 1910 and 1960, even though the actual optical designs varied little. Mostly, the advent of anti-reflection coatings and very minor changes to internal design of the barrel reduced flare a little and increased contrast. If I had a choice, I would probably aim for a silver barreled one, from the 40's or 50's but even the early Leitz objectives I have tried out are very good.

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