From Magic to Microscope
The Signature of Backbone
A ‘living fossil’ with unusual appearance, the horsetail or Equisetum was a herb frequently used in early modern European medicine for a wide range of diseases, from wounds and blood fluxes to coughs and bowel problems. Particularly, as shown in the illustrations from Wolfgang Fabricius’s thesis Ἀπορημα βοτανικον de Signaturis Plantarum
(1653), its stem with joints and knots very much resembled a backbone and was considered to benefit bone health.
Botanical teaching diagram showing Equisetum pratense. By S. Dalton.
Botanical teaching model of a horsetail plant (Equisetum), made of wood and papier mâche. By Hagenbeek Wageningen, 19th Century. Wh Ac 5368
‘Joynted and Knotted’
With the rise of microscopy in mid-seventeenth-century England, the horsetail became a popular metaphor for describing microscopic observations. Seeing the hairy legs of insects for the first time under microscopes, many people found these very similar to the stems of horsetails. Microscopist Henry Power described the legs of a flea he observed through microscope in Experimental Philosophy (1664): ‘Her legs all joynted and knotted like the plant call’d Equisetum or Horse-tayl, and all hairy and slit at the ends into two toes’.
The same horsetail metaphor was used in Robert Hooke’s lavishly illustrated Micrographia (1665) to describe the two horns of a gnat: ‘...Two long jointed horns, tapering towards, the top, much resembling the long horns of Lobsters, each of whose stems or quills, were brisled or brushed with multitudes of small stiff hairs, issuing out every way from the several joints, like the strings or sproutings of the herb Horse-tail’.
For many early modern scientists, the idea of signatures inspired observation and experiments. Physician Henry Power, author of the first English book on microscopy, *Experimental Philosophy*
, believed that in order to discover God’s ‘eminent signatures of Divine Providence’ we need use telescopes and microscopes to read the distinctive characters of things. One could ‘never discern those distant, or minute objects by Natural Vision, as we do by the Artificial advantages of the Telescope and Microscope’.
Title page of Henry Power, Experimental Philosophy (London, 1664). Image from Wellcome Collection, Public Domain.
With the concept of signatures, an idea became deeply inscribed into the vocabulary of modern science that the observable, distinctive features of natural things could reveal their essence. In an 1852 letter to Michael Faraday, chemist Christian Friedrich Schönbein used the idea of ‘signatures’ to demonstrate the importance of colour to chemical science: ‘In more than one respect the colour of bodies may be considered the most obvious “signatura rerum (signature of things)”, as the revealer of the most wonderful actions going on in the innermost recesses of substances, as the indicator of the most elementary functions of what we call ponderable matter’.
Michael Faraday's microscope slide used in his lecture on gold sols in 1858. Wh.0654
Faraday’s experiment on sols (colloidal suspensions) showed the importance of colour as ‘signatures’ for understanding chemical phenomena. Gold sols had been known to alchemists in the 17th century, but Faraday was the first to present a scientific paper on their properties and preparation. The preparations of gold sols that Faraday studied were ruby-red in colour, but Faraday discovered that he could turn the preparation blue by adding certain salts. Faraday concluded that ‘known phenomena seemed to indicate that a mere variation in the size of [gold] particles gave rise to a variety of resultant colours’.