What Are These Spencer Eyepieces For?

[KurtM]

Seller says he thinks they're for aligning an old Spencer phase contrast scope (Model 15 was the first Spencer offered with phase, I believe). But if that's so, why would they come in a pair?

[lorez]

I wish I could tell you more, but I wonder about the phase application. All the phase telescopes I am familiar with are focusable. Did you get any additional details ?

I think these would be difficult to use because of the apparently small lens opening, but maybe not. I guess getting your pupils centered is about the same, regardless...

lorez

[apochronaut]

I think the fact that there are two of them , is simply because the seller has two of them. These were made for Spencer polarizing microscopes and are pinhole eyepieces , used primarily for viewing the interference pattern .
Someone with more experience in using a polarizing microscope than I , would be able to elaborate more.

[gekko]

I think the fact that there are two of them , is simply because the seller has two of them. These were made for Spencer polarizing microscopes and are pinhole eyepieces , used primarily for viewing the interference pattern .
Someone with more experience in using a polarizing microscope than I , would be able to elaborate more.

Interference patterns in a polarizing microscope are usually viewed using the Bertrand lens, so they are at the back focal plane of the objective (where phase centering telescopes are focused for centering the condenser phase annulus), so the seller may not have been too far off.

[apochronaut]

I guess the Spencer Lens co. must have made a mistake then, when they catalogued pinhole eyepieces in the section for monocular polarizing microscopes.

[Charles]

These are pinhole eyepieces specifically for confocal pol use...although they could also be used for phase alignment.

[gekko]

I guess the Spencer Lens co. must have made a mistake then, when they catalogued pinhole eyepieces in the section for monocular polarizing microscopes.

Sorry, apochronaut: I probably didn't word my post very clearly: Charles is correct. What I meant was that both the conoscopic view in a polarizing microscope and the phase ring in a phase objective are at the back focal plane of the objective, so what works for viewing the interference pattern also works for centering the phase annulus.

[Charles]

These are pinhole eyepieces specifically for confocal pol use...although they could also be used for phase alignment.

Gekko is right, I meant conoscopic pol use.

[KurtM]

Thanks all. Kinda makes me wish I knew what the heck that means.

[gekko]

Thanks all. Kinda makes me wish I knew what the heck that means.

KurtM, please forgive give me if this is gibberish, but I'll attempt an explanation as best as I can, hoping that someone will explain it much better. The conoscopic view, a term usually used in petrology (analysis of rock sections using polarized light) is the interference pattern formed at the back focal plane of the objective; it is the Fourier transform of the image, so it is, in the spacial frequency domain, the diffraction pattern that represents the image.
The objective's back focal plane is also where effective spacial filtering can be applied, although often, for convenience, its optical conjugate, the condenser aperture is used; for example, reducing the size of the condenser iris reduces resolution (low pass spacial filtering), darkfield stop (high pass spacial filter), etc.; the objective's aperture itself being a low pass spacial filter that places an upper limit on resolution for any given light wavelength.

[Edit:]I found the following link which I think explains the principle much better than I can:
http://cns-alumni.bu.edu/~slehar/fourier/fourier.html
As an example, blocking what they call the DC term (corresponding to the 0th diffraction order) is what we do when we use darkfield illumination, so we can think of a darkfield stop as being analogous to a capacitor used to block the DC component of an electric current. Fourier transform = diffraction pattern which is what the conoscopic view shows (back to the original question).

[KurtM]

Gekko, no worries, I was mostly joking around, I know I have Professor Google right here if I need answers -- but at the same time THANK YOU for your considered reply! I'm aware that petrologist microscopy is highly specialized and not many people know much about it. Having started this conversation, I suppose it's the least I can do (for myself) to look into all this and try to understand.

It always makes me want to run and hide whenever hear the "back focal plane of the objective" mentioned, because I'm never been able to figure out what's being talked about. Yes, I've tried removing eyepieces and gazing down focuser tubes, but what I see never equates to what I read about? I have a phase telescope if that helps, as well as a shelf full of books (that I have even read). I would kinda like to get this figured out one of these days, maybe it'd help me understand N.A., which might help me get even more out of my microscopes.

I've said it before and I'll say it again: I'm just happy as heck I don't have to understand all this in order to enjoy microscopy. But I'd still like to. Thanks again!

[gekko]

Hi Kurt,
I feel your frustration. I will try to reply after I had time to clarify things in my own mind so I can try to be more lucid. I have just found a fair "Interactive Tutorial" on the Zeiss website. I say "fair" because they could have done a much better job of explaining the relationship between resolution and aperture using their diffraction grating object, but it is still far better than anything I can say to explain it: I hope it helps a bit.
http://zeiss-campus.magnet.fsu.edu/tuto ... flash.html#
If you wish, please ignore my very lame attempt below at explaining this further.
The white hexagon (when you select white light) in the center is the image of the opening of the condenser diaphragm, and represents the zeroth order of the diffraction pattern of the line grating. This is the part that is blocked when one does darkfield illumination. The colored spots on either side of the center are the 1st and (for the coarse grating) 2nd order of the diffraction pettern of the line grating, and those form the detail of the image (diffracted light). If they are blocked, the lines will not be resolved. The finer the diffraction grating (lines closer together), the farther apart the diffraction orders will be. As the grating spacing becomes smaller and smaller, the diffraction pattern gets wider and wider until the first orders miss the objective aperture, and the lines can no longer be resolved by that objective. Unfortunately the tutorial does not show this (only two grating spacings are provided, and the objective numerical apertures available for selection are enough to resolve both of those gratings). The tutorial does show you that, with the coarse grating where 1st and 2nd orders enter the objective aperture, blocking the 1st orders results in an image with half the line spacing (i.e. as though we are looking at the finer of the two gratings), which is what one would expect if you think of the visible 2nd orders as now being a 1st order (since the actual 1st orders were blocked the the spacial filter).