If you grew up as a kid in the 1970s and ever any read comic books, you doubtlessly came across ads like the one above for a product called "X-Ray Specs." For a mere $1 you could get your very own pair of x-ray glasses, enabling you to see through walls like Superman. If you were a boy of a certain age back then, you were probably somewhat intrigued by the gross-out possibility of viewing the bones in your own outstretched hand, but were positively captivated by the prospect of being able to see through women's clothes.
Today these types of products are dismissively categorized as "novelties." The fact that they did not work as advertised is therefore taken a given. But not back then. At least not in my elementary school. No one ever ordered a pair, I don't think. But as we hit 4th, 5th and 6th grade, these x-ray glasses came up in conversation more and more frequently, especially when girls were nearby.
But what were they, really? As it turns out, they had nothing whatsoever to do with real x-rays, of course. They were really just an oversized pair of glasses with plastic frames and cardboard lenses. Each "lens" consisted of two layers of cardboard with a feather sandwiched between them. A small hole was then punched through the middle. And the wearer viewed objects through these holes. The veins of the feathers were so close together that they diffracted incoming light, causing the user to receive two slightly offset images. And that produced the (blurry) illusion of x-ray vision apparently. (Would it surprise you to learn that the guy who invented these was the same man who marketed brine shrimp to children as anthropomorphic "Sea Monkeys"?)
In fairness, looking at that ad above in more detail now, there is more truth in the copy than I remember. It does term the whole thing a "hilarious optical illusion," and says that you "seem" to see the bones in your hand. I didn't remember these ads being anywhere near that equivocal. But if I had ordered a pair, I am sure that I would not have found the whole underwhelming reality anywhere near as "hilarious" as the makers suggest.
As an aside, did you notice how the ad treats the more prurient "look through women's clothes" aspect? There's a very clear drawing of a bug-eyed boy staring at a woman's silhouette under her dress. But then the text below reads, "Look at your friend. Is that really his body you 'see' under his clothes?" 'His' body? Was that some sort of attempt to soften this aspect? (Better to encourage young boys to try to look at each other unclothed, I suppose they thought, than their matronly school teacher?)