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A spectrograph, or as Captain Maitland called it, a spectroscope, was an instrument with lines that, according to Ian Chesterton, represented "the emission wavelengths." John, a mineralogist, used a spectrograph to, as Carol Richmond put it, "take a reading of the minerals in the vicinity" of the Sense Sphere. The graph that it produced told Ian that the area contained oxygen, hydrogen and sodium. The First Doctor also found that the planet contained molybdenum, which was hard to read on the graph due to it being mixed up with the lines. (TV: "The Unwilling Warriors")