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Alex

I can only speak to your question with the applied knowledge of a
semi-professional photographer and amateur woodworker, augmented by the wit
and wisdom of the list.

A highly polished black surface (like a clean new black car) will want to
absorb all visible wave lengths by virtue of being black, but actually acts
like a mirror because of the polished surface, especially for specular
highlights (sun, spotlights etc) with sharp edge transfer between highlight
and background.  Note at least one sextant handbook suggests you can breath
life back into a dead rear-silvered sextant mirror by painting the back
black.

A white, highly polished surface wants to reflect all visible wavelengths.
Why it is not effective as a mirror I do not know in theory.  If I had to
hazard a practical guess, even it it was as effective as a polish black
surface, it would give off too much light for the human eye to adjust for,
especially in the context of (ratio to) its surroundings. And/or highlights
would, by ratio, be too close to background noise to show a marked
difference. I will have to experiment with stopping down a lens to place the
value of a polished white object a stop or three above the film's threshold
and see what happens. (I can tell you glass objects, particularly art work
by glass artists that are translucent, transparent, and opaque in one piece
are a real challenge to photograph.  A wine glass, for example, likes to be
mildly front lighted from very broad and even sources, and back lighted to
give it form and volume. Upshot, get glass thick enough it absorbs a lot of
light.)

As to reflections, I suspect a black bottom with clear oil would still give
a double reflection to some degree (depending quite a bit on the polish of
the bottom).  When photographing an object on tinted glass/plastic in the
studio it matters as to whether the tint is all the way through the
glass/plastic groundplane, or if the glass/plastic ground plane is clear and
surface coated.  A uniform tint will yield one discernable reflection, while
surface coating yields two reflections.

Additionally, if using a mirror (generally Plexy) on location to backlight
(rim or hair light) a front-lighted subject by kicking the sun off the
mirror (and a filter is placed over the mirror to warm up the color of the
back light) it will double the effect of color added from the filter (as
opposed to just placing it over a light source) as the light passes through
it once on its way to the mirror, and again on it way from the mirror to the
subject.  So a dark oil will filter out most of the light to and from the
bottom of the vessel before it reaches the observer.

Coincidentally I assembled an artificial horizon from Karo and Canola oil
yesterday evening.  A laser pointer seems to produce only one discernable
reflection.  That surprised me until I remembered the filter-over-mirror
effect.  When I added glass (roughly parallel to the oil surface) over the
vessel, I got 3 discernable reflections.  The top and strongest reflection
from the top of the glass, the middle from the Karo/oil surface, and the
source of the bottom/third I have not yet spent the time tracking down.
(Light billiards.  No time and sun simultaneously for field testing yet.)

As you probably know, the more elements you add to a camera lens, the more
light kicks around inside the barrel (element off element) resulting in
ghosting, flare, and loss of contrast.  Hence multi coated lenses to reduce
reflection off the elements.  To the best of my knowledge, only a recently
developed "miracle glass" offers 100% transmission.  Which by default would
mean no reflection? (I sure don't need that in may patio sliding doors.
Imagine the carnage on the way to the Weber;-)

Possible alternative to water and oil.  As an amateur woodworker I play with
aniline dyes.  They are produced in water-, alcohol-, and oil-soluble forms.
My gut says I will be happiest with a heavy grade motor oil spiked with a
healthy dose of black oil-soluble aniline dye.  An alternative might be a
tube or two of black universal tint from a Menards paint department, but I
hate to think of all those huge pigment chunks scattering light willy nilly
and settling to the bottom as toxic waste/sludge.  (As I recall, you
described the Karo/Canola oil as a "deadly" mixture off list.  Not sure the
FDA would agree, but who trust them anymore?  I would certainly rate motor
oil and aniline dye as far worse for your digestive track and carpet than
Canola oil and water-soluble Karo. )

To sum it up under the working title of "Rants on Physics and Chemistry BY a
Dummy:"

The clear oil is the major reflective surface.

It also protects the Karo from air so it does not skin, solidify by
evaporation, or possibly crystallize like an old bottle of honey.  And it
doesn't produce water vapor to fog up the glass when you Davis greenhouse
hits sunlight.

The dark Karo provides a dark background for the clear oil so it more
effectively reflects highlights while absorbing ambient light sources.

The thicker the dark Karo layer, the more light is absorbed, so less light
that can travel to the bottom and resurface as a double reflection off the
bottom of the vessel.

An added benefit is the Karo is pretty viscous, so you are not setting up
little seiches as you transport this deadly liquid from indoors to the
patio.

Probably more thinking points than a concrete answer, but that's may best
shot at the moment--until I push "send" and then realize how seriously
flawed the above is.

Bill


> This question was discussed a lot in October 2004,
> and I want to add something.
>
> Recently I made many experiments with Davis art horizon
> sold by Celestaire.
>
> I tried various combination of Caro syrup and oil,
> Caro syrup alone, and oil alone.
> (Caro syrup was recommended on this list).
>
> The best "combination"
> seems to be pure vegetable oil, without any syrup.
> I really don't understand why this sirup was proposed.
> (To make the bottom darker? What for? And if this is
> indeed useful, why not to paint the bottom of the horizon
> vessel black with ordinary paint?
>
> The syrup hardens when the weather is cold (it was 23 F last
> weekend), always has impurities and causes double reflection
> unless the oil layes is extremally thin. Such thin layer of
> oil is usually not enough to smoothen the surface of sirup.
>
> I Have not tried the dark (mahine) oil yet, but why it is
> considered useful to have dark substance in general?
>
> Alex.
>
> P.S. I DO understand why Bauer and others recommend sextant
> mirrors painted black on the back side (to prevent second
> reflection from that back side) but with liquid filled art horizon
> with plastic botom it is different: there is no reflection
> from the bottom.

   

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