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Science Stories

OPTICS

1672

Isaac Newton

announces his

experiments

with prisms

ABOVE: Newton’s sketch of a beam of light passing through prisms
from a hole in his window shutters. This was proof, he claimed, that
white light already contains the colours of the rainbow

cience’s greatest legend was launched church window, or candles make diamonds found it “a very pleasing divertissement,
Salmost 300 years ago when, shortly vibrate with colour, we see light that has been to view the vivid and intense colours
before he died, Isaac Newton (1642– modified during its journey between the produced thereby”.
1727) told several friends that he had original source and our eyes. After a while – at least, this is how his
conceived his theory of gravity beneath a In contrast, Newton maintained that the story runs – he decided to investigate fur-
tree in his country orchard. Like the colours of the rainbow are already present ther, and placed a second prism in one of
attribute of a saint, Newton’s apple has in what appears to be white light, and he set the coloured rays. When the light passed
become the iconic symbol of a scientific out to devise a way of confirming this ex- through the glass unchanged, Newton
genius forever voyaging through strange perimentally. In 1672, he claimed that two claimed that he had disproved the modi-
seas of thought alone. ordinary prisms were enough to provide fication theory of the French philosopher
Yet when he was elected a fellow incontrovertible proof that he was right. René Descartes.
of London’s Royal Society in his late Like the supposed flash of inspiration Newton’s experiment was dramatic,
twenties, Newton was acclaimed not under the apple tree, Newton dated his key but was it crucial? Arguments raged for
for his theoretical daring, but for his experiment in optics back to 1666, a year years. His bitter enemy Robert Hooke ac-
practical expertise. in which he spent much of his time at his cused Newton not only of stealing his own
A skilled craftsman, Newton had cre- Lincolnshire home after plague forced him research, but also of failing to produce
ated an impressively small yet powerful to quit Cambridge. In a deceptively chatty definitive proof – there were, he insisted,
telescope, painstakingly polishing lenses letter, Newton explained that he had placed alternative explanations. European
and mirrors designed to reduce distor- a prism in a beam of light shining through experimenters raised another serious
tion. Views of his own past were less clear a small hole in his window-shutters, and objection: Newton’s results were impos-
cut. When he presented his first paper on sible to replicate, because he had left out
prisms in 1672, Newton glossed over the vital details such as the type of glass and
Newton’s results
sidetracks and false hopes that are inevi- the dimensions of the prisms.
table in any protracted research project. Few in England dared contradict
Instead, he made it appear that one single were impossible to Newton’s authority, yet as an Italian critic
demonstration – a “crucial experiment” – protested, it would be strange if “in places
vindicated his ideas and showed beyond replicate, because where experiment is in favour of the law,
doubt that his rivals were wrong. he had left out the prisms for doing it work well, yet in
According to the theory prevailing places where it is not in favour, the prisms
then, light is altered in its passage through vital details for doing it work badly”. Perhaps Newton
a medium such as air or water: when the is not as transparent a hero as he might TOPFOTO/ALAMY
sun shines through the stained glass in a seem. Words: Patricia Fara

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