A brief critical summary of the principal contributions published on the theory of the subject is presented, followed by the report of a series of experiments designed to test the methods which have been proposed. It is concluded that while relative measurements can be made with sufficient accuracy, absolute values cannot be relied on as yet. The results emphasize the necessity for careful attention to such details as the construction of the camera, the preparation of the sample, and the characteristics of the x‐ray film and of the microphotometer.