Experimental infrared spectroscopy

The infrared spectrum was discovered by Herschel in 1800. He found that a thermometer was heated by invisible radiations when he placed it beyond the red end of a solar spectrum formed by a glass prism. From this primitive discrimination between the visible and invisible, experimental physicists have refined their procedures until now the $15,000 cm−1$ compass of that invisible spectrum is resolved into some 30,000 separable subdivisions of $1/2 cm−1$ width, with prospects of resolving it into 200,000 subdivisions in the near future.