Duboscq colorimeter, 2000

Museum of the History of Sciences, Ghent University, Ghent Belgium. Invented in 1870, Duboscq colorimeters were sold through the mid-1900s. One sample container holds solution of known concentration (Ck); the other holds solution of the same solute of unknown concentration (Cu). The pathlength for the known solution (Lk) is fixed; that for the unknown solution (Lu) is adjusted until the intensity of the (colored) light appears the same when viewed through the eyepiece at the top. The product of pathlength and concentration is constant: Lk X Ck = Lu X Cu, allowing one to determine the concentration of the unknown solution.