On light. Third course, On the beneficial effects of light, delivered at Aberdeen in November, 1885 / by Gabriel Stokes.

  • Stokes, George Gabriel, 1819-1903.
Date:
1887
Catalogue details

Licence: Public Domain Mark

Credit: On light. Third course, On the beneficial effects of light, delivered at Aberdeen in November, 1885 / by Gabriel Stokes. Source: Wellcome Collection.

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    This question has been subjected to experiment by several persons, and notably by the late Dr John Draper, and at his suggestion by Dr Gardner, who worked with all the advantage of a Virginian sun. Instead of trusting to coloured glasses, the light transmitted by which is of a compound character, and usually includes invisible rays from one or both ends of the spectrum, they used sunlight which was reflected horizontally into a perfectly darkened 100m, and passed through a prism placed in the window. The spectrum thus formed was not it is true pure, but still the colours were not greatly mixed, and it could only be quite neighbouring parts of the spectrum that overlapped. The result was very decided; the evo- lution of oxygen by green leaves placed in watei charged with carbonic acid, and the turning green of leaves which had germinated in the dark, and so before exposure presented only yellow seedling leaves, both one and the other were strongest about the brightest part of the spectrum, about the greenish yellow, and from thence decreased in both directions ; the blue and violet rays in particular, which act so powerfully on most photographic preparations, being almost wholly if not wholly inactive in producing the phenomena now under consideration. The accordance between the parts of the spectrum which produce these two effects respectively makes it probable that the