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In the beginning of episode 5 of Brideshead Revisited there is a scene where one of the characters uses a "Universal Projector" to show some slides. It can currently be watched here, about 2m30s in.

What's confusing me is there are a couple times when the projector slide is flipped over to show a new image, when I would have though the image would just end up upside-down.

slide flipped
(Not the best example since the gif only shows one projection but it's clear from the narrative that two are there.)

On closer inspection there's no room for a light to project from behind the slides and I'm not sure the slides are transparent.

Here again the slide is first rotated to put it upright, then flipped over to show a different image: https://i.imgur.com/LMuPoGV.mp4 (too big for SE)

What type of projection system is this?

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  • Perhaps a completely fictional one? On the other hand maybe the "cartridge" is split into sections, each of which holds an image, but e.g. only the "top" half is projected at any time, so flipping the cartridge is required to deal with the other half.... – twalberg Jun 18 '20 at 18:53
  • Re, "I would have though the image would just end up upside-down." Assuming that card is opaque (read about "opaque projectors, in the answers below), then it probably has two different images on it—one on either side. Which way is "up" on one side of the card has nothing to do with which way is "up" on the opposite side. – Solomon Slow Jun 19 '20 at 17:17
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This is an opaque projector (Wikipedia). The image of the illuminated photo print, postcard, painting, etc., is projected through the lens to the projection screen. A slightly different geometry, for greater magnification, is used to project art onto screens or walls for tracing or copying the image. Artigraph is a brand of these copying opaque projectors.

According to the Wikipedia article, projectors used to be divided into two classes: diascopes, where light shines through transparent slides; and episcopes, where light reflects off opaque prints. Thus, opaque reflectors are also known as an episcopes.

Episcopic projection is much less efficient (i.e., requires a more powerful light source) than diascopic slide/transparency projection. And because the power of light sources directly corresponds to the heat produced by them (especially with the pure incandescent or flame generated light used back in the day), and the need to protect the opaque image from the generated heat, this limited the usefulness of episcopic projectors. For equivalent power light sources, opaque projectors were dimmer than slide projectors, and therefore could not accomodate larger screens so large numbers of people could view them without being too dim.

The site Luikerwaal.com is dedicated to Magic Lanterns (Wikipedia), with lots great examples, including several episcopes and epidiascopes (convertible, able to show opaque or transparent sources). Interestingly, they have an image of an old advertisement for a "post card magic lantern", suggesting that at the time of the ad, all projectors were types of magic lanterns.

enter image description here
Advertisement for a "post card magic lantern", from Luikerwaal.com

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  • @SolomonSlow great point. Added. – scottbb Jun 19 '20 at 21:15
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It is called a "Lantern Slide" projector. Going back, perhaps 100 years, positive images were prepared, not on film, but on glass plates. These were projected for an audience via a projector that initially illuminated via gas light or other combustible. The next iteration used electric light bulbs.

This projector is also commonly called a "Magic Lantern".

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  • How did the image get from the slide to the screen when it was on the opposite side of the light as the screen? – xiota Jun 19 '20 at 2:08
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    Some designs are "opaque" projectors, These brightly illuminate a picture or drawing. A mirror reflects an image of the picture which traverses a projector lens. Opaque projectors are still in use by artists and are commonly used in school classroom or meeting room. These are a variance of an overhead projector. – Alan Marcus Jun 19 '20 at 2:20
  • When I worked at Harvard University just a decade ago, the basement of the engineering school had cupboards full of old magic lantern slides from lectures years past which no one really knew what to do with but did not want to throw away. One of the older professors emeritus told me that he remembered using them. – mattdm Jun 19 '20 at 4:03

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