Staining: Making the Invisible Visible

The mounting rung ended on an awkward fact: a good mountant matches the refractive index of the tissue, and in doing so makes an unstained specimen almost invisible. You have removed the glare and the detail with it. The fix arrived from an unexpected direction. The coal-tar dye industry of the 1860s and 70s — the same chemistry as the synthetic alizarin and Para red blueprints — produced hundreds of brilliant aniline dyes for textiles, and biologists discovered that they stained some cell structures and not others. Paul Ehrlich built a career on which dye went where, and in 1884 Hans Christian Gram found a sequence that split all bacteria into two groups by the structure of their cell walls. A stain is not paint. It binds where the chemistry lets it bind, so WHERE it lands is itself the information.