At first glance, the title of this post probably appears quite paradoxical. After all, the very definition of an object being visible is seeing light coming off of the object! At second glance, you might think the title is referring to infrared radiation or ultraviolet radiation, both of which were discovered in the early 1800s and which lie outside of the visible spectrum of light. But this is also not the case: the tenebroscope is a device that was introduced to demonstrate that visible light is, in a sense, invisible!
I was recently asked to write a retrospective on invisibility for a scientific journal, and I did a search for “invisibility” in the scientific literature in the 19th century to see how that word was used then. I already knew that it was used to describe infrared radiation and ultraviolet radiation, and also to describe particles that are too small to see with the naked eye; this time, however, my search turned up the curious reference to the “Tenebroscope.”
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