The Claude glass is a usually sepia tinted, convex, small, transportable mirror, and it was very popular in the 18th century. When an artist or enthusiast found a particularly picturesque landscape they would take out the Claude glass, turn their back on the view and view it through the reflection of the glass. Through the looking glass the viewer would be able to see the landscape as if it was a painting by the 17th century artist Claude Lorrain. Though this seems to be a rather quaint practice it tells us something very significant about how nature was perceived and portrayed back then.
Now, Claude Lorrain may or may not have actually invented the Claude glass—the story goes that it was when he accidentally sooted a wine glass that he observed the effect and subsequently used it for his drawings and painting—but the fact was that Claude Lorrain's style was still idolized and seen as the proper form of landscape painting even a century later, which points out how strong the traditions of art were held. Painters even used a yellow varnish to emulate the patinated varnish of the old masters in order for their paintings to look right.
The Claude glass was used because nature as a spectacle wasn't valued as much in itself other than perhaps as an inspiration to an artist. An exact painting of nature as it is perceived by the naked eye would have been considered "low art" and very uninteresting. Especially the mundane and pedestrian landscapes of Northern Europe in comparison to the history-soaked landscapes of Italy. Instead, the artist would rather forget about the particularities of a landscape and create an idealized landscape loosely based on what was seen. They would do little (or big) tricks with the perspective, creating a wideangle effect or combining several perspectives for dramatic effect in a cosmorama. They would add a shepherd and his flock for a pastoral flavor, or ruins and figures from ancient Greece for the then ever-popular classical look. And in this, the Claude glass came in handy.
Even to non-artists the Claude glass was an easy way to enhance the spectacle of landscape and imagine that they were in fact viewing a beautiful painting instead of vulgar nature. Still, optical devices like the Claude glass, or the camera obscura, were novelties, and thus some people have likely only used it as a toy, marvelling at how their perception of the world could be changed so dramatically.
The attentive reader has probably already noted that while writing about the artist, the proper spelling is 'Lorrain' without an 'e' at the end, and while writing about the looking-glass, the proper spelling is 'Lorraine' with an 'e.' I suspect this is a consequence of 18th century English spellers unaccustomed to transcribing French, but I don't know. The common short form, 'Claude glass,' as used in this writeup, dispels any such spelling dilemmas.