The concentration of plant substances such as the chlorophylls and carotenes can most readily be quantitatively measured by colorimetric methods. Colorimetry also offers a rapid method for the quantitative determination of organic substances such as sugars, oxidases, and inorganic ions as magnesium, iron and phosphates, in which colors can be developed when in combination with certain other ions in solution. The personal equation in such measurements has long been known to contribute a source of error where such methods have been used. An extended bibliography of which part 2 of this paper is composed, has several references which show that the use of photoelectric cells in this regard improves the accuracy of colorimetric investigations. A simple photoelectric colorimeter is here described and experimental data presented in support of certain specifications of construction and operation
