What air, water and crystal offer the eye

An alchemical text transmitted under the name of the Swiss-born physician Paracelsus (1493/94–1541) brings three substances together in a discussion of looking into crystal. Air, water and crystal are easy enough to distinguish by touch. Water yields to pressure, crystal retains its shape, and the air between an eye and an object usually becomes noticeable only through something moving in it. The writer begins his comparison with a different sort of experience.

A comparison with a precise limit

The crystal is a figure of the air. Whatever appears in the air, movable or immovable, the same appears also in the speculum or crystal as a wave.

– Pseudo-Paracelsus, The Coelum Philosophorum, 1894, p. 14.^[1]

He expressly specifies sight. A piece of clear crystal can show the eye something of the transparency of air while possessing a firm surface. Water in turn can return an image which alters as soon as its surface is disturbed. Looking at the three substances side by side produces different answers to the same question. What does one actually see when looking into a transparent body, and where is the image that appears to be present there?

A surface for sight

The text belongs to a historical understanding of sight in which a crystal could serve as the setting for a vision. The qualification about the eye therefore deserves attention. It gives a daring comparison a precise reach. An observer can have air, water and stone together in the same line of sight. A movement of the head brings first a surface and then something beyond it into focus. The familiarity of that experience helps explain why a writer thinking about the almost impalpable air should turn to an object as firm as crystal.