Seeing Through Skin

For two hundred years after Borel, progress was negligible. The instruments were crude, the images dim, and the skin itself was the obstacle. The problem is one of physics. When light strikes the surface of dry skin, most of it bounces straight back — scattered by the rough, keratinised surface of the stratum corneum. This surface glare overwhelms the eye, making it impossible to see anything beneath.

The first step toward solving this came from the world of precision optics. In 1878, Ernst Karl Abbe — a German physicist working alongside Carl Zeiss, the renowned microscope manufacturer — introduced the use of cedar oil as an immersion medium for microscopy. In modern optical terms, this works through refractive index matching: when the refractive index of the medium between the lens and the specimen closely matches that of the specimen itself, surface reflection is dramatically reduced and light passes through rather than bouncing back. The stratum corneum has a refractive index of approximately 1.54, close to the refractive index of glass (approximately 1.52), so an oil or fluid with a similar index effectively renders the outermost layer transparent.

In 1893, the Hamburg dermatologist Paul Gerson Unna applied this principle directly to skin. In a paper titled Diaskopie, Unna demonstrated that immersion oil used with a microscope could render the epidermis translucent. He recognised empirically that the upper layers of the skin were blocking light from entering, and that applying oils or other fluids to the surface could overcome this barrier.

Unna did not describe his findings in the language of refractive index — that quantitative optical framework would come later — but his practical insight was exactly the phenomenon that modern optics would explain as index matching. This was not about polarisation (a different optical phenomenon entirely, and one that would not enter dermoscopy for another century). It was about creating an unbroken optical pathway from the lens, through the fluid, through the skin surface, and into the tissue beneath.