Frederick William Herschel (1738 - 1822)

Sir William Herschel (Figure 4) was born in Hanover, Germany, but emigrated to England at the age of nineteen with his sister Caroline. He was of course best known for his astronomical work - especially the discovery of the planet Uranus - but he was also the first to prove that there were radiations beyond the visible spectrum and is credited with the discovery of infrared.

Figure 4 (left). Sir William Herschel.

Herschel was interested in learning how much heat passed through different coloured filters he used to observe the Sun and noticed that filters of different colours seemed to pass different levels of heat. Herschel thought that the colours themselves might contain different levels of heat and so in 1800 he devised a clever experiment to test his hypothesis.

Herschel directed sunlight through a glass prism to create a spectrum and measured the temperature of each colour. He used three thermometers with blackened bulbs (to better absorb the heat) and placed one bulb in each colour while the other two were placed beyond the spectrum as control samples (Figure 5). As he measured the temperatures of the violet, blue, green, yellow, orange and red light he noticed that all of the colours had temperatures higher than the controls and that the temperature of the colours increased from the violet to the red part of the spectrum. After noticing this pattern, Herschel decided to measure the temperature just beyond the red portion of the spectrum in a region apparently devoid of sunlight. To his surprise, he found that this region had the highest temperature of all.

Figure 5 (above). Herschel's experiment to measure the temperature of each colour in the spectrum.

Herschel performed further experiments on what he called the "calorific rays" that existed beyond the red part of the spectrum and found that they were reflected, refracted, absorbed and transmitted just like visible light. He termed this radiation 'ultra-red' (it was not until 1881 that Abney described the radiation more correctly as 'infrared').

Unfortunately some infrared photography web sites wrongly attribute the near invention of photography to William Herschel - but it was his son John Herschel, also an astronomer and scientist, that has that distinction. On 22 January 1839 John Herschel heard of Daguerre's work on photography from a casual remark in a letter written by Beaufort to Margaret his wife. Without knowing any details, Herschel was able to take photographs himself within a few days. Fox Talbot visited him on the 1 February to discuss their separate experiments in photography, the occasion being recorded in a letter by Herschel's wife:

I happen to remember well the visit to Slough of Mr Fox Talbot, who came to show Herschel his beautiful little pictures of Ferns and Laces taken by his new process. When something was said about the difficulty of fixing the pictures, Herschel said, "Let me have this one for a few minutes" and after a short time he returned and gave the picture to Mr Fox Talbot saying "I think you'll find that fixed".

This was, of course, the beginning of the hyposulphite method of fixing the image - the basis of all modern silver halide photography.

References:

• Herschel, W., 1800, "Investigation of the powers of the prismatic colours to heat and illuminate objects", Phil. Trans. 255.
• Herschel, W., 1800, "Experiments on the refrangibility of the invisible rays of the sun.", Phil Trans. 284.

< Isaac Newton

Johann Wilhelm Ritter >