Optical phenomena and the properties of elements and compounds in relation to light have always been a major interest for Gladstone. This comes out quite early in his career, and in a variety of forms. Thus in 1854 he lectured at the Royal Institution on "Chromatic Phenomena exhibited by Transmitted Light." In 1855 there were "Notes on some substances which exhibit the Phenomena of Fluorescence", and in 1856 on "Some Dichromatic Phenomena among Solutions". In 1858 he drew attention to the use of the prism in qualitative analysis ("Quart. Journ. Chem. Soc.", 1O, 79), and discovered distinct lines in the absorption spectrum of didymium, a substance long afterwards resolved by Auer von Welsbach into the two elements known as praseodymium and neodymium. A little later he studied the absorption spectrum of the atmosphere, and found that the Fraunhofer lines varied according to the time of day, and that the change must be due to some constituents of the earth's atmosphere. In this research he was joined by Sir David Brewster, and together they produced a paper on the lines of the solar spectrum in 1860. The most important work of Gladstone in this direction was the long series of observations on the refraction and dispersion of liquids, which originated with a study of the "Influence of Temperature on the Refraction of Light," (Phil. Trans., 1858) and followed by "Researches on the Refraction, Dispersion, and Sensitiveness of Liquids," ("Phil. Trans.", 1863).