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effect reaches a maximum, and that measurements confined to the visible portion of
the spectrum failed to locate this point.
10398903;a1
Figure 13.2 Marsilio Landriani (1746–1815)
Moving the thermometer into the dark region beyond the red end of the spectrum,
Herschel confirmed that the heating continued to increase. The maximum point, when
he found it, lay well beyond the red end – in what is known today as the 'infrared
wavelengths'.
When Herschel revealed his discovery, he referred to this new portion of the electro-
magnetic spectrum as the 'thermometrical spectrum'. The radiation itself he sometimes
referred to as 'dark heat', or simply 'the invisible rays'. Ironically, and contrary to
popular opinion, it wasn't Herschel who originated the term 'infrared'. The word only
began to appear in print around 75 years later, and it is still unclear who should receive
credit as the originator.
Herschel's use of glass in the prism of his original experiment led to some early
controversies with his contemporaries about the actual existence of the infrared
wavelengths. Different investigators, in attempting to confirm his work, used various
types of glass indiscriminately, having different transparencies in the infrared. Through
his later experiments, Herschel was aware of the limited transparency of glass to the
newly-discovered thermal radiation, and he was forced to conclude that optics for
the infrared would probably be doomed to the use of reflective elements exclusively
(i.e. plane and curved mirrors). Fortunately, this proved to be true only until 1830,
when the Italian investigator, Melloni, made his great discovery that naturally occurring
rock salt (NaCl) – which was available in large enough natural crystals to be made
into lenses and prisms – is remarkably transparent to the infrared. The result was that
rock salt became the principal infrared optical material, and remained so for the next
hundred years, until the art of synthetic crystal growing was mastered in the 1930's.
52
Publ. No. 1 557 544 Rev. a121 – ENGLISH (EN) – October 6, 2005
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