Anyone know where to buy semi-silvered glass?

Thanks very much for explaining it to me. I too like to learn how things are made. But sometime I feel the need to limit the number of projects I'm working on!

Best wishes,

Chris

I suspect that it won't work for him. What he needs is a semi-silvered mirror -- something like 50-75% of the light passes through, and the rest bounces. It goes between the eyepiece and the objective lens set up so light from a lamp to the side bounces down and passes through the objective lens to illuminate the object under study, and the image which comes back has most of the light going on up to the eyepiece, with the smaller percentage bounced back to the light, where it does nothing useful. What would be even better would be an arrangement where all of the light from the lamp goes through the lens, and all which bounces back from the object under study goes up to the eyepiece, but physics is not quite so cooperative.

Enjoy, DoN.

Thinking about it quickly, I think the efficiency might be better using a plain sheet of glass and making use of the total internal reflection effect. What do you think?

Best wishes,

Chris

[ ... ]

Hmm ... at what angle do you get total internal reflection? IIRC it is a function of the index of refraction. And while binocular prisms get TIR at 45 degrees, the light enters and exits at close to 90 degrees, thanks to the design of the prism. If you have the light entering at 45 degrees on one face, it will be bent towards the other face so the angle will be steeper than 45 degrees.

And -- *if* you got TIR with the glass (say you made a full

45-45-90 prism to get the entrance angles right) you would get nothing through to your eyepiece.

So I think that the semi-silvered is the better choice.

Hmm ... the light is not focused where it passes through the mirror, so you might be able to take a first-surface mirror and scribe through the silvering in a pattern to get something like 30-45% reflection, and then you might be able to use it. I've seen scribed patterns in mirrors in SLR cameras, with the CDS cell for metering behind the scribed section.

Enjoy, DoN.

41.8 degrees for light travelling from glass into air. This would mean that the mirror would need to be at an angle of less than 41.8 degrees to the horizontal.

The only disadvantage as far as I can see is that if the light emerging from the lamphouse is horizontal, it won't be travelling quite vertically downwards after being reflected. But you could correct this by tilting the lamphouse and its lenses down slightly.

Best wishes,

Chris

Thanks for the offer, but I discovered that the design is intended to use plain glass. I called a few glass suppliers and couldn't find a piece the right size. In the end I found another identical microscope on eBay cheap. It has a few issues but with the pair I'm sure I can build a working microscope.

Nevertheless, thanks very much for the offer.

Best wishes,

Chris

In the end I tracked down another Watson "Standard Metallurgical" microscope on eBay. It's in better condition, but still has a few issues. With spare parts from the first I should now be able to build a microscope in good working order.

For the benefit of anyone who comes across this thread in the future, the reflector in the Watson "Standard Metallurgical" is plain glass, not a semi-silvered mirror.

Many thanks,

Chris

A scribed mirror is a fascinating idea! But it seems like it is done by total internal reflection in this microscope. I've yet to completely absorb the intricacies of the optical system, but it's interesting to note that the reflector pivot is not directly above the centre of the objective lens. It's slightly in front on the lens, i.e., closer to the light source.

I also need some lens-removing tools. Not sure what they're called. I'll probably post a message about those at r.c.m in the next few days.

Thanks for the help.

Best wishes,

Chris

You must mean spanners. Edmund Optics, Thorlabs for some pricey ones. Ebay for a budget type.

Often the rings are hardly tight, and can be turned out with the tip of a tiny screwdriver on one side, or the filed-down tips of needlenose pliers on both sides. Give it a try.

What's the official name for these spanners?

I'm reluctant to try it. I don't want to cause any damage.

Many thanks,

Chris

Spanner wrench or adjustable spanner wrench. The first fixed type is a thin hollow tube with tiny projections on the end to fit the holes or slots in the retaining ring. This is better at deeply recessed locations. The second is the adjustable gadget with interchangeable tips.

See:

and search for "spanner".

See:

and search likewise.

Really, just nudge it with a jeweler's screwdriver or dental pick and see if it doesn't back right out.

On Wed, 05 Mar 2008 23:48:29 +0000, Christopher Tidy wrote as underneath my scribble :

Plain glass transmits IR through (greenhouse effect) - so I think your 45 plain glass is doing exactly what a proper cold mirror does - its probably much cheaper but loses more of the visible reflected light on the way together with all the IR, the function is the same - to avoid permanent damage to the viewers retina, especially if on a production line with halogen lamp sources etc...... standard mirrors reflect IR and surface coated mirrors also reflect the UV components as well as IR, so a cold mirror has to be multicoated to get rid of the UV as well as pass through the IR. Various informed guesses here! Charlie+

O.K. Looking under Edmund Scientific, I did not find them last night, but looking under Edmund Optics tonight, I find them. Try:

complete with a link to download the (one-page) manual in PDF format.

Indeed so. Or make a special spanner as I described in the article which I just posted.

Enjoy, DoN.

Try searching for 1/2 aluminised glass. Might yield better results.

Steve R.