The need to measure accurately the area of several thousand apple leaves in connection with a study of the deposit of spray residues led to a search of existing methods of leaf area measurement. The several methods described in the literature failed to satisfy the requirements, inasmuch as accuracy in area measurements was apparently to be secured only by methods too lengthy to be of use in this study. Existing methods for measuring leaf areas include the following: (1) Measurement of the leaves along their axes, the area obtained by multiplying these figures being corrected by a factor; (2) planimeter measurements; (3) tracing or otherwise transferring the leaf outline to paper of known weight per unit of area and later determining the area of the tracings by their weight; (4) leaf -punch methods, in which definite areas of the leaf surface are removed by a punch or die. The first of these methods is not sufficiently accurate, since variations in leaf shape among individual leaves of the same species is sufficient to introduce gross errors into the calculation. The second and third methods are extremely time-consuming, depend largely for their accuracy on the skill and dexterity of the person using them, and are adapted only for measuring the areas of small numbers of leaves. Circles or other geometric figures cut from the leaf surface with a punch or die are subject to errors arising from the obvious fact that the leaf presents neither a uniform surface nor an interior structure free from gross tissue differentiation. The use of the photoelectric cell in a variety of ways in recent years led the writer to consider the possibility of such a device for measuring leaf areas, inasmuch as the area would be a direct function of the amount of light intercepted if the leaf were placed in the path of a beam of light. While a device was being perfected, a short news article appeared describing a similar apparatus, built by Withrow, and it was learned that a paper describing a similar apparatus had been presented at the Boston meeting of the American Society of Plant Physiologists. Definite information concerning the construction of this apparatus could not be secured, however, and the present apparatus has been developed independently, and has been used satisfactorily for the past season. 1 Publication authorized by the Director of the Pennsylvania Agricultural Experiment Station, as Technical Paper no. 677.