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 Post subject: Need some advice on a dichroiscope
PostPosted: Fri Mar 14, 2014 1:33 pm 
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Hi Folks,

Some while back I acquired the little instrument depicted below from a retired gemologist.

Attachment:
Dichroiscope.jpg
Dichroiscope.jpg [ 209.75 KiB | Viewed 905 times ]


I believe this is an old GIA model? Also I suspect from the weight that it's not a calcite type but rather the split polarizing filter variety?

No matter what stone I put up to stone end, (tourmaline, topaz, iolite, quartz) the little round window is not divided into halves but stays resolutely round and uniform in colour. Moreover, when I put a polarizing filter to the stone end and turn the dichroiscope, I would expect to see two semi-circles alternately darkening and lightening. But again, nothing happens. The viewing window stays round and uniformly lit. No dividing line is visible whatever.

Has anyone else encountered this? Do these things typically fall apart or otherwise degrade over the years? There's no rattle to suggest that something has come unglued, but unfortunately it's a sealed unit so I can't get inside it to see what's going on.

(If this one's shot I'm thinking of making one with the lens from an old 10x at one end and a split polarizing sheet at the other)

Cheers all
Hans Durstling
Moncton, Canada

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 Post subject: Re: Need some advice on a dichroiscope
PostPosted: Fri Mar 14, 2014 2:43 pm 
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Hi Hans. It looks like a calcite dichroscope to me. A question which I hesitate to ask...Have you tried looking at stones through either end?

Maybe the thing has come unstuck inside. There is a very good thread somewhere by Gene which walks through repairing one and sticking it all back together...Unfortunately I can't seem to find it. (help Julie or Barbara...search engine champs)

Unscrewing the end and getting the insides out might tell you more. let us know what you find.

best

Frank


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 Post subject: Re: Need some advice on a dichroiscope
PostPosted: Fri Mar 14, 2014 2:45 pm 
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Sorry Hans just noticed that you said it doesn't unscrew...scratch that idea then :-)


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 Post subject: Re: Need some advice on a dichroiscope
PostPosted: Fri Mar 14, 2014 3:42 pm 
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Have you tried looking through both sides?
Don't laugh.
I always look through the wrong end of stuff. :oops:
Finally i got smart and I mark the end to look through with a scratch or sticky dot.


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 Post subject: Re: Need some advice on a dichroiscope
PostPosted: Fri Mar 14, 2014 4:51 pm 
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Oh I'm not laughing. Somewhere in reading up on this before posting the question I came across the quip, "there should be a nut on each end" - so no, henceforth I shall never make the wrong-end mistake, not after a mnemonic like that.

Cheers,
Hans

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 Post subject: Re: Need some advice on a dichroiscope
PostPosted: Sat Mar 15, 2014 12:32 am 
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The GIA dichroscope is a Calcite type. But even with the Calcite in place it hardly weighs anything. It is just a small piece like 5x5x9mm or so.

I have never tried to open mine. It should not make any difference which end you look into. I think you look into the round end and the square window appears double. But you were quite right to try a polarizing filter. If you don't see the two images blinking back and forth then the birefringent element is missing.

This would be true also in one made with polaroid filters. It does not matter which end you look into because the ordinary and extraordinary rays split the same way in both directions. In one direction you see to squares and in the other you see two circles.
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"there should be a nut on each end"

I DO like that one. :lol: :D

If you feed it polarized lighting you don't get semicircles you get overlapping circles. (presuming its calcite. You do get semicircles with polaroid.)

It is difficult to cut polaroid with out inducing damaging strain along the cut.
Since I know you are a faceter, you should get a block of calcite and replace the calcite element.

Somebody once made and described a calcite dichroscope with glass wedges laminated to the calcite to make it zero deviation. I think it was Stewart of London and late nineteenth or early twentieth century. Replicating that is number 457 on the list of things I want to do if I live long enough. :lol:

The "optical" calcite used in dichroscopes is water white, totally colorless. Polaroid is not. It has some blue and green shelves in its spectrum.


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 Post subject: Re: Need some advice on a dichroiscope
PostPosted: Sat Mar 15, 2014 10:58 am 
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Thank you G4 - your information confirms my thinking (there's no polarizing happening whatever) and deepens the mystery. If the calcite had come loose in some way I'd expect to hear a rattle inside - but there's nothing. Looks like dissection may be in order.

As for the knife-strain cutting polaroid sheet, I'm thinking I should be able to cut first, then grind away the strain zone. I'll keep you posted.

Cheers & thanks again
Hans-D

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 Post subject: Re: Need some advice on a dichroiscope
PostPosted: Tue Mar 18, 2014 1:18 pm 
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All of those Dichroscopes seemed so solid, that I am sure there is a liberal dose of Loctite holding threads together. You will need to apply heat to open the thing I am pretty sure. They may be "pressed" together.


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 Post subject: Re: Need some advice on a dichroiscope
PostPosted: Tue Mar 18, 2014 6:12 pm 
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G4 - By dint of vigorous twisting with a serrated-jaw pipe wrench I managed to at least loosen the non-stone end of the dysfunctional dichroiscope. (Ah yes, delicate gemmological instrumentation). But I was reluctant to dismantle the thing entirely until I had a functioning fallback, which I built over the weekend, and yes indeed it work. The building process (split polarizing film type) got extensively photographed & I'll put it up here as a photo story probably before the day is done.

Cheers
Hans Durstling
Moncton, Canada

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