Photo-chemistry / by S.E. Sheppard.

  • Samuel Edward Sheppard
Date:
1914
Catalogue details

Licence: In copyright

Credit: Photo-chemistry / by S.E. Sheppard. Source: Wellcome Collection.

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    of light,” Lucretius and others held a quasi-tactile theory as to the action of vision, regarding the ocular beams as species of levers by which the eye felt or “ palped” the object. On the other hand, Pythagoras supposed that vision was excited by small particles emitted by visible objects. Opposed to these views of light as a primary quality was the opinion of Aristotle, who considered that light was not a substance in any sense, but a peculiar quality or action (energeia) of a pellucid medium, which intervened between the object and the eye. Colour was a minor, light a major activation of this medium. These three theories, the normative theory of Plato, the corpuscular of Pythagoras, and the undulatory of Aristotle, occur in one form or another in all subsequent speculation. § 2. Alchemists and Iatro-chemists. The alchemists, preoccupied with the endeavour to isolate the assumed abstract principles or qualities of material sub- stances, with the aim of transmuting metals, seem to have paid little practical attention to the action of light. Similarly with the iatro-chemists, who by liberal experiment on the human frame, founded rational therapeutics, naturally break- ing many eggs in preparing their omelette. And as to the alchemists, their dialect or jargon differed from ours, but as Berthelot remarks1 : “ Les opinions au.xquelles les savants tendent k revenir aujourd’hui sur la constitution de la matiere ne sont pas sans quelque analogie avec les vues profondes des premiers alchimistes.” § 3. Observations on the Chemical Action of Light. The influence of light on the formation of the green colour of plants, and the converse bldaching in darkness, was noted by Aristotle (350 b.c.) ; and Vitruvius remarked on the bleach- ing of pigments by light (roo B.C.). The writings of the 1 M. Berthelot, Les Origines de f A/chi//lie, Preface, p. 15. Paris (Steinheil), 1S85.
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