an early apparatus for measuring the depth of tint in a fluid by comparing it with a standard fluid; glass cylinders are immersed in each of two cups, one containing standard fluid and the other the fluid to be tested; on looking through the cylinders, the tints are equalized by raising or lowering the cylinder in one cup; the extent of this raising or lowering is indicated on a scale and gives the exact difference in tint. [Jules Duboscq]
