From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "Michael Burr" 
>
>Vladimir said:
>
>> I think that you can not make hair (wire) thinner or thicker at will.
>> It is related to the slit width and what you are blocking is slit image
>>  where it is sharpest ( thinnest). If you have fixed slit and use to thin
>> hair, some light will spill around it and nulling the zone will be less
>pronounced
>> ( contrast).
>
>Ahh, I see. I am constantly amazed at how much there is to learn.
>
>Now the question becomes how thin the slit should be, and how to go about
>making it. I've read in the archives about methods for making pinholes, but
>not much for slits. A pinhole would work pretty much the same, should it
>not? And perhaps a pinhole would work better with a tester that has the
>source and hair moving together on an arc, because the slit would allow
>light to pass through on more axes.
>
>Probably I'll try a pinhole first, layering aluminum foil and driving a
>needle through the layers.
>
>Thanks!
>MTB
>
>

Hi
 If you consider the wire ( alias hair ) test to be like
a slitless tester, you can vary the source slit width to get the best
contrast at the wire. The idea is that the image of the slit forms on the
wire. If the slit is too narrow, you will block too much light. If the slit
is too wide, too much light will get around the wire and you won't see as
good a shadow.
 Of course, you can use more monochromatic light and adjust
the slit width to produce intended interference stripes to measure from.
 I make a simple adjustable slit by mounting two edges
on a small parallelogram frame. By pushing on a corner of the parallelogram
and holding the other side steady, the slit can be finely adjusted, with a
set screw. I made the entire frame with brass strips I got from a hobby
shop. I bent and soldered these together. I found that the angle of about
120 degrees works well. The natural spring of the brass is enough for the
normal range of slit sizes.
Dwight

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