How did Herschel discover infrared radiation in 1800?

Answer

A thermometer placed beyond the visible red end of the spectrum registered higher temperatures.

While passing sunlight through a prism, Herschel noted that an area outside the visible light band, specifically past the red portion, caused a thermometer to register the highest heat, indicating an invisible form of energy he termed calorific rays.

How did Herschel discover infrared radiation in 1800?
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