Eye spy...

.... something beginning with chromatic lenses. Invented by a Mr. Dolland in 1758, said lenses are made to minimise chromatic aberration (where a lens can't properly refract all the wavelengths of colour in the same point.) Photographers are grateful, even if they are shooting in black and white.
My hands might look mucky, but I did give them a thorough scrubbing after bike fettling.

Many thanks to Laurie54 for hosting the ingeniously themed 18th. century inventions mono monday.