I have my reading glasses sitting on the desk in front of me. They are 2.00 power glasses and they are sitting upright and with the ear pieces towards since I just took them off and set them down. Over my shoulder is a well lit window in an otherwise dim room. A clear image of that window is floating about 2" back from the lens. I can move my head side to side and up and down and the image moves around. It is not on the surface of the lens, or at least appears not to be. It’s like a little hologram.
The wonders of imaging science.
It was by such optical legerdemain that they created the Tanagra Theater image – a wonderful illusion that I researched recently. You get a perfect miniature image that you can apparently move around and see from different angles. Since your eyes see them from slightly different angles, the image appears to be 3D, like a hologram. It was heavily exploited in the 1920s into the 1960s, but has been pretty much forgotten since then. You could only show the image to a very small audience across a narrow set of angles, so it wasn’t cost-effective (much like real Holographic movies).
The biggest thrill with Tanagra theater was that you could use it with nude subjects. There were miniature “nudist colonies” and the like at the 1933 Chicago world’s Fair and the 1937 Great Lakes Expo.
the sole survivor of this is Dolphina the Mermaid at Bimbo’s 365 Club in San Francisco. She’s been there since the 1930s (I know she’s still there – I’ve corresponded with the current club owners)
This was a recurring character on The Chris Isaak Show.
The final regular character, Mona, appears lying nude on her stomach on a revolving circular bed in the basement of Bimbo’s, a club Isaak frequents. Through an optical illusion created with mirrors she’s made to look as if swimming in an aquarium, and being called a mermaid (despite having human legs). Only once does she sit up, revealing her toplessness. She typically talks only with Isaak, acting as his conscience or a sounding board for him.
I think the most amazing lab demonstration I saw in a university science class was this: the professor set up a lens aimed at a distant window, with a frosted screen on the other side of the lens, and moved them until there was a sharp image of the window focused on the screen. He had us look at the screen from the other side, that is, with the screen between us and the lens. We concentrated on the image. Then he pulled the frosted screen away, and we realized the image was still floating there in space.
Of course, this only works if you view the screen such that the lens is directly behind it and in line with it from your point of view. But it still instantly cemented in my mind the idea of images floating there in space, whether or not you planted something there for them to hit.
Hey I remember this from high school. One of the physics teachers had a dish that was mirrored on the inside with a matching domed lid with a hole in it. He’d put a small object in it and it would appear floating above the hole. it was pretty cool.
The wonders of Edmund Scientific!
It’s called a mirascope. For nine bucks you can have one too.
I remember a trip to Gatlinburg, TN where this effect was popping up everywhere. There was a “crack” in the outside wall of the Ripley’s Believe it or Not museum where you looked in and saw (something) floating, at least one giftshop had the toy on display, and the video arcade had this:
We seem to be discussing a couple of different imaging technologies. The mirascope works by using two parabolic mirrors that reflect an image of something from one focus to another. The deal with the lenses sounds different, and the Dolphina example doesn’t give a clear enough description to be sure.
Yes. I mentioned that in my article about the Tanagra Theater*. The idea also showed up in one of the Columbo mysteries, only there they used a woman dressed like an actual mermaid – no nudity on prime-time network TV.
Here’s an abbreviated version on the Oxford University Press blog:
You’re quite correct. (I did an article on the Miracle optical illusion, too). Annd you correctly state the setup – the Mirascope consists of two parabolic reflectors placed so that the focal point of each coincides with the apex of the other, only there isn’t actually an apex for the top one – it’s cut away so you can see inside.
What’s very good about both the Tanagra Theater setup and the Mirascope setup is that there is virtually no distortion and no color separation, so the image looks perfect, even if you move around and look at the image from different angles. I note that both cases use parabolic mirrors and no lenses (lenses are more expensive in large sizes, and tend to have some chromatic aberration. Mirrors are less expensive and don’t have the chromatic.) There’s at least one other similar illusion where you have a seeming perfect image using a parabolic mirror – that’s the case where you place an object two focal lengths away from the focal point of the parabolic mirror (usually very well-lit, and upside down). A very perfect life-sized image will appear to be floating directly above it, so real you are tempted to reach out and touch it. They used to have one of these set up at Boston’s Museum of Science.
The Bimbo’s Mermaid probably used a Tanagra Theater-like setup of parabolic mirror and paired right-angle inverting mirrors, which accounts for the perfection of the reduced image. There are other, cheaper ways to produce a similar illusion using lenses or lens substitutes. One uses a negative lens instead of a positive lens. The other uses a positive lens and an inverting mirror. The sideshow version uses, as a lens, a fishbowl (often filled with clear mineral oil, rather than water). The image is distorted and has chromatic aberration, but these are excused as the effects of her being underwater. A slightly more elaborate version uses two fishbowls, but you can do it with one. What I love about it is that it’s very cheap and easy to set up, and is very compact. It’s much more forgiving than the elaborate Tanagra Theater illusion, and doesn’t require the expensive large parabolic mirror. I constructed one myself when researching this, and got to view my wife as a mermaid.
The Exploratorium in San Francisco has (or had?) one of these two. (It’s been many years since I was last there.)
In the 1960’s there were some billboards around town that used some kind of imaging illusion that might be related. The billboard showed a person’s face. From the point-of-view of drivers driving by, the person seemed to be looking right at you. As you drove by, the eyes seemed to follow you. Creepy! (I’m kinda thinking the Exploratorium had a display similar to this too.)
I don’t know the billboards you’re speaking of, but your description sounds like the masks you can see near the beginning of Disney’s Haunted Mansion in both Orlando and Anaheim. The masks appear to swivel to follow you. It’s a very creepy illusion. The “masks” are actually inside-out hollow faces, the thing you would get if you took a mask and hung it on a wall with the concave inside facing toward the viewer rather than the convex outside. Because of your expectations, you perceive the mask as being convex-out, as you normally would se one. But as you move around the sight doesn’t really look like a statis convex mask – it looks like a convex mask that is swiveling to follow you. But it’s really a stationary concave mask
Watch this video. The section explaining the masks starts at 4:14
Yes. The Exploratorium in San Francisco had (and perhaps still has) exactly that exhibit. I’ve assumed that the billboards were constructed similarly, just larger. They also had that exhibit with the bowl with some small object floating above it, and you reach in to touch it and there’s nothing there.
The Exploratorium is largely a museum of perceptual exhibits of various sorts. It was founded by Frank Oppenheimer, brother of J. Robert Oppenheimer, both of whom were blacklisted by the House Un-American Activities Committee.
I’ve been to the Exploratorium several times. It’s a more involving and grittier science museum than most others I’ve been to, unless it’s changed since I’ve been there. Lots of things I haven’t seen elsewhere.
One of my college professors was fond of starting a lecture by swinging a baseball bat through the image of a lightbulb produced in this way.
And it sounds like, even though @mixdenny 's glasses are obviously lenses, the effect he’s seeing is from a reflection from the surface, so they’re actually acting as mirrors, given that the real window is “over his shoulder”.
Yes – my explanation given above for his illusion assumed it was surface reflections responsible for the imaging.
There’s a practical application for such imaging, too. There’s a device called a radiusscope that consists of a microscope equipped with a movable stage that has a position gauge mounted to it, and is also fitted with a beamsplitter that projects an image of a crosshair onto the surface being studied. You put a curved piece onto the stage (it can be either concave or convex) and move the stage until you find the surface location with its projected crosshair. Set that location as “0”, then move the stage until you find a second location where you see the crosshair. You will see a perfect image of the crosshair “floating in space”, not on a surface. You’re looking at the image of the crosshair created by reflection from the surface, and its location will be two focal lengths from the surface, which is equal to the radius of curvature. You just read the value off your translation stage. You don’t need an absiolute zero – just the distance between those two points.
It’s a neat device. We built an automated version of it, so all you had to do was push a button to get the radius. Then we went on to build similar devices for surfaces that were toric or aspheric and didn’t have a single radius of curvature. We’d measure them all.
I knew that. 
This comes up just often enough that I remember and conflate the two
Here’s a simple paper version that you can download, print and assemble:
Sounds similar to the principle of a Reflector sight - Wikipedia