The English poet Thomas Gray climbed onto a hilltop near the village of Keswick in 1769, and instead of simply taking in the view of the misty meadowland below, he took out his Claude glass, a convex, tinted mirror, something like a lady’s compact, which artists and tourists kept close at hand in the late eighteenth century, convinced that the reflection of a pretty view was usually prettier than the view itself. In order to see the landscape in his mirror more clearly, Gray turned his back to the sight and aimed the Claude glass back over his shoulder, framing up ‘a picture, that if I could transmit it to you, & fix it in all the softness of its living colours, would fairly sell for a thousand pounds,’ he wrote to a friend.

…

Like Gray, fastidious European aesthetes set out in pursuit of the sweetest scenery, armed both with clear colored glass filters that tinted their views pink, green, or blue, and with black-tinted Claude glass mirrors, which distorted the landscape, while seeming to improve it.