The German physicist Joseph von Fraunhofer had begun to examine light by passsing it through a prism, which deflected the rays. The angle of deflection depends on the precise colour of the light, and the colour is synonymous with the wavelength of the light. He noticed that each mat- erial burns with a flame of a distinct colour and wavelength, and therefore the wavelength can act as signature for the material. For example, sodium always burns with an orange colour that corresponds to a wavelength of 0.5893 millionths of a metre (or 5893A).
A few years later, Gustav Kirchoff and Robert Bunsen had built a spectroscope, an instrument with an in-built prism designed to measure the wavelength of any light. Working in their Heidelberg laboratory one evening, they saw distant flames from the direction of Mannheim. They turned their spectroscope towards the fire to see if they could detect what was burning and thereby identified wavelengths that correspond to barium and strontium. The Mannheim fire made them realise that spectroscopy, the analysis of light, could be used to identify the constituents of any hot, luminous body. They soon turned their instrument towards the sun.
The science of solar spectroscopy developed gradually, and scientists identified one by one the elements that constitute the sun. Then a problem arose. In 1868, the French astronomer Jules Janssen travelled to India to observe the eclipse and detected rays of sunlight with a wavelength of 5876A, just a little shorter than the sodium wavelength. Initially, he put the discrepancy down to mismeasurement, and believed that he had detected sodium. The British polymath Sir Norman Lockyer also measured a wavelength of 5876A, but instead of labelling it sodium, he postulated that the wavelength indicated a new element, which he named helium, from the Greek word "helios", meaning Sun. His hunch was confirmed in 1895 by the discovery of helium on Earth, identified by the Scottish chemist William Ramsey.
Auguste Comte had been proved utterly wrong. Not only was it possible to study the chemical composition of the Sun, it was possible to discover a hitherto unknown element, even though helium was already present on Earth.