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I have a couple of C. Baker objectives of x40 and x100 that are the cross type instead of annulus type. I tried to make a cross to fit the swing out filter arm on the condenser and set it up so it matched the x40 perfectly. I could see a grey coloured image but it was not very clear. Am I barking up the wrong tree with the positioning of the homemade cross on the underneath of the condenser at the iris? The cross slits I made are the width of a scalple blade and when looking down the scope without an eyepiece they look about twice the width of the cross in the objective. It would be useful to know what the width of the slits should but I haven't found anything on the Web yet that tells me.

Ruptor, I believe (but I may be wrong, in which case I hope someone will correct me) that the slits should be no wider (no thicker) than the the cross on the phase plate in the objective when viewed with the eyepiece removed; slightly narrower (thinner) should be your target, I think. I think the location on the filter ring below the condenser diaphragm is correct (it should not be too far below, but as close as reasonable to the diaphragm). And, of course, the condenser should be correctly positioned. Just a note: a centering telescope (or a do-it-yourself substitute) that allows you to enlarge the image of the back focal plane of the objective will be helpful in ensuring that the two crosses are aligned correctly, etc. as it might be difficult to see this clearly with the unaided eye.

Thanks RWR. I expect you can guess my next question. How on earth do I make slits narrower than a scalple blade. At the moment the image with a full 100W halogen and the current cross I made is very dim. If I make the slits narrower will I actually be able to see anything?

Well, I was dreading your next question . The largest one on my condenser has a width of about 1 mm, and the smallest I would guess about 0.3 mm (very roughly as I cannot get close enough to it), so they are pretty narrow. And, consequently they do block most of the light. But if not all of the direct light passes through the cross, the phase contrast system will not be as effective, yet may still give more contrast with phase objects than regular bright field, depending in how wide the slits are. Again, please take what I say with a few grains of salt, since I'm mostly guessing. I hope someone will chip in and correct me.

Ruptor and RW and all,

This is a most interesting discussion. I have not seen the C. Baker 'cross' style phase set and any searches seem to bring C. Baker into the explanation as a footnote. Ruptor, I would like to see, if possible, some photos of your cross style phase objectives where the cross is visible. I will admit I have more questions than answers at this point. Thanks.

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A pixel is worth a thousand words but it takes a thousand words to explain a pixel. g2b2

Well that is a job and a half. The only way I can see the cross inside the objective is to stick against my eye ball the wrong way round. I mean with the sample end of the objective on the eye. I don't know how I can get a photot of the cross but if I think of a way I will do it.

To be able to see it properly, to compare the size of the cross in the objective with that of the condenser "cross" diaphragm, and to align the two, you really need to use a centering telescope or perhaps you can improvise one: a close focusing telescope of some sort that can focus in the range of 150 to 200 mm and enlarge the object by several times. The microscope should be focused on something on a slide the usual way, the slide moved to a clear area, the condenser in the uppermost position, then the eyepiece is removed, and one can look down the tube through the telescope and focus the telescope on the cross in the objective which should be in focus simultaneously with the image of the condenser diaphragm. The matching of the size of the cross in the phase plate of the objective should be with the image of the condenser diaphragm cross as seen looking down the tube with the eyepiece removed, not with the actual physical size of the condenser diaphragm cross.

An interesting historical note: the C. Baker phase contrast system using the cross phase plate was designed to allow a single substage diaphragm to be used with all objectives, according to an old book I have.

Yes I read that somewhere RWR but it followed with " each lens needs to be lined up before use" so I think that was the down side since a circular system doesn't require it I guess since once centred all lenses would be centred. It suprises me that they couldn't have arranged the crosses to stop in the same place when the objectives were screwed in to the mount or some other fix to prevent alignment being required with each objective change.
Attached is a picture of a green laser pen stuffed up the x40 lens and projected on to the side of a fridge. A shocking picture but the horizontal bar of the cross can just about be seen about a third of a way up the picture and the vertical bar is in the centre. Ignore the diagonal line and the tracks up the left side.

Attached Thumbnails

 

A circular system indeed does require aligning them. Neither the objective turret nor the condenser turret are sufficiently accurately made to obviate adjustment for each objective.