violet end to the red end. This was not entirely unexpected, since the
Italian researcher, Landriani, in a similar experiment in 1777 had obser-
ved much the same effect. It was Herschel, however, who was the rst to
recognize that there must be a point where the heating effect reaches a
maximum, and that measurements conned to the visible portion of the
spectrum failed to locate this point.
Figure 16.2 Marsilio Landriani (1746–1815).
Moving the thermometer into the dark region beyond the red end of the
spectrum, Herschel conrmed 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 electromagnetic spectrum as the ‘thermometrical spectrum’. The
radiation itself he sometimes referred to as ‘dark heat’, or simply ‘the invi-
sible 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 conrm
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 infra-
red would probably be doomed to the use of reective elements exclusi-
vely (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
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