Color-comparison tubes, sometimes referred to as Nessler tubes, have been the standard equipment for making colorimetric measurements for many years. Tubes of this type are shown in Fig. 14.1. Their use has largely been replaced, however, because of the convenience of photoelectric and spectrophotometric methods. Precise work with color-comparison tubes requires that tubes of matched size or bore be used in order to comply with Lambert’s law. The chief difficulty with their use is that standard color solutions are seldom stable, and every time a determination has to be made it becomes necessary to prepare a series of fresh standards. This adds greatly to the labor and time required. Another objection is that all comparisons are made by eye, and the “human error” involved is often considerable because sensitivity to different colors varies. Furthermore, the analyst is required to interpolate values between standards.