Spectroscope with comparison scale, c 1860-1870.

UNITED KINGDOM - SEPTEMBER 22: Sir William Crookes (1832-1919), the English chemist, used this portable direct-vision spectroscope made by John Browning of London. The compact instrument could be employed to observe the spectrum of faint light sources. Lacking the narrow slit found on most spectroscopes, instead light enters from the left side of the instrument where it is dispersed by a series of triangular glass prisms. Cemented together apex to apex, so forming a straight bar of glass, they form a detailed spectrum that is viewed using the eyepiece on the right side of the spectroscope. Crookes applied spectrum analysis to his examination of the deposit from a sulphuric acid factory, which led him to the discovery of a new metallic element, thallium, in 1861. (Photo by SSPL/Getty Images)