The Oxford English Dictionary defines episcotister as “An apparatus for admitting light into a darkened room by means of adjustable discs.” Merriam-Webster (m-w.com) defines it as “a device for reducing the intensity of light in known ratio by means of rapidly rotating opaque and transparent sectors.” Dictionary.com defines it as “a disk with a sector removed that when rotated in front of a light source allows the periodic passage of flashes of light: used in studying the motion of a body.” The Wikipedia article on perceptual transparency defines it in passing as “a rotating disk that alternates open and solid sectors.”
With each successive pass it becomes a bit clearer. Take a metal disk. Cut out half of it at the diameter, or cut out a pie-piece-shaped quarter of it and, diametrically opposing, cut out another quarter, so you have two quarters solid and two quarters cut away. Maybe leave a little bit of rim to hold it together. You now have something that can be used as an episcotister if you spin it in front of a light source, with its axis to one side of the light source (especially with a lens) so it alternatingly blocks and reveals the light. Spin it fast enough and you get the fan effect: it becomes transparent, although dimmer.
This makes me think of the sound of the word: /ɛ ˌpɪ skə ˈtɪs tɹ̩/. Notice how your lips and tongue move to block and open the passage of air and sound: the vowels and syllabic liquid /ɛ ɪ ə ɪ ɹ̩/ let it through, like light; the consonants /p sk t st/ block it. Easy to hear where it’s vowel and where consonant. But speed it up (repeating) so that the stops are passing by a few hundred times a second and it will all blur together. Indeed, that’s how you can make musical tones: percussions repeated rapidly enough that they blend all together.
And this is where episcotisters really make their difference. I’ll quote Nick Burlett, who suggested I taste this word: “The human visual system can perceive flicker below about 50–60 Hz (or 50–60 on-off transitions per second), but above that rate the light source appears constant (the flicker “fuses” into a continuous experience). Modern movies are shot (nearly universally) at 24 frames per second, which is below the threshold of flicker fusion, meaning that we would perceive a flickering image were it not for the episcotister in the projector. Rather than displaying 24 images per second, the projector displays 48 or 72… flashing each frame of the movie onto the screen two or three times before advancing.”
That’s right. If you were watching a movie at just 24 flashes of the light per second, you would see it as 24 flashes of the light, just as you would hear 24 “t”s a second as a succession of “t t t t t” et cetera. If you watch a fan pass across a light slowly, you can see the blade. But speed it up and you hear not “t t t t t” but a note; you see not a blade again and again and again but a steady transparent slight dimness. Drive across a cattleguard slowly and you feel every bump; do it quickly and it’s just a quick vibration like a pass of a back massager.
The fascinating thing here, of course, is that the frame actually only changes once every 1/24 of a second, the movement only moves 24 times a second, but because you’re seeing it twice or three times for each frame, somehow the jerkiness of the movement is elided. Our brain and eyes establish perceptual continuity with the light, and the motion is inferred by the mind as continuous and so seen as such. (There’s a lot of “this must be this” in the brain’s visual processing.)
The other term for this device, as it’s used in movie projectors, is rotary disk shutter. But episcotister is more fun. It comes on strong with a taste of Episcopalian, though it has no particular relation to English bishopric, and a contrary Scot, though it is no more Presbyterian; it also gives you some pisco, that South American grappa, again irrelevant but beyond control. There is also a bit of an echo of taster and of twister. But it comes (as you may have guessed) from Greek. The source is ἐπισκοτίζειν episkotizein ‘throw darkness or shadow’ (note from the accents that the long syllables were the first and second-last syllables in the Greek); that comes from ἐπί epi ‘upon’ and σκότος skotos ‘darkness’, which we see also in scotomata.
We always think we need to cast light on a subject in order to see it more clearly. Perhaps, perhaps. But sometimes we need to cast a bit of darkness on a subject in order to make more sense of it.