Trying for Dark Ground photos

[exmarine]

Hi members,
I have been trying to obtain dark ground on my microscope which has not been beneficial at all. See photos. I can get an object in view but can not get it to show the dark ground effect. I am using a paraboloid glass condenser with oil touching the back side of the slide, then using a X40 objective then up to X90 oil immersion objective.
Any advise from members would be most welcome. Thank you.

[gastrotrichman]

I don't have a lot of darkfield experience, but I wonder if your darkfield condenser is centered. For discussion, see: https://www.olympus-lifescience.com/en/ ... ieldsetup/

[75RR]

I don't have a lot of darkfield experience, but I wonder if your darkfield condenser is centered. For discussion, see: https://www.olympus-lifescience.com/en/ ... ieldsetup/

Very useful link. Thanks!

[MicroBob]

Hi,
some points to check:

- A paraboloid condenser offers a hollow cone of light with an upper and a lower n.a. E.g. 1,2 to 1,4. The lower value has to be somewhat higher then the n.a. of the objective. The better everything fits together the smalle this difference can be.
- There are different dark field condensers, also some with low n.a. values
- These condensers are made for a specific slide thickness.
- The hight of the condenser has to be set perfect so the hollow cone hits the object
- For an 40:1 achromat the usual high power oil immersion dark field condensers work very well, try this first
- For high poweer dark field you need an oil immersion objective with iris to lower the n.a. of the objective somewhat
- The condenser has to be centered as mentioned before
- The slide has to especially clean to get a nice image

To me it looks as if you are not far away from getting this to work. First use the 40:1.

Bob

[exmarine]

Thank you to everyone who has given advice its well appreciated.
I will endeavour to carry out what has been mentioned and get back to you.


Meanwhile I used a metal stop to see what would happen. Here are a couple of shots.