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Figure1.3 Plate from W. Herschel's infrared paper (1800).
where our eyes' sensitivity leaves off, at a wavelength of about 400 nm. Sir William
Herschel also examined this part of the spectrum with his prism-and-thermometer
apparatus but found no perceptible temperature increase when his thermometer was
placed in the dark space next to the violet band of light. This led him to conclude
that there were no rays beyond violet:
“By these last experiments I am sufficiently convinced that no rays,
which exist outside the violet, can have any provable force, neither
in the forms of light, nor of heat, and that these two forces prolong
along the prismatic spectrum, and ends there where the farthest violets
disappear.” 3
Johann Wilhelm Ritter (1776-1810), a contemporary of Herschel, discovered
near-UV light in 1801. Ritter used a similar prism and screen apparatus to create a
rainbow pattern. But instead of a thermometer, he used silver chloride crystals as
a detector. The crystals darkened more rapidly when exposed to the near-UV light
from the prism than they did when exposed to visible colors of light. Herschel's
thermometer was not sufficiently sensitive to detect near-UV light, which led to his
3 W. Herschel, “Experiments on the refrangibility of the invisible rays of the Sun,”Phil.Tran.Roy.
Soc.90, 284-347 (1800).
 
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