D O C U M E N T 4 3 8 1 9 2 7 4 1 9 IV. What is the best practical radiation measurement procedure (X-rays, gamma) in qualitative and quantitative respects? I do not have the necessary knowledge to answer this question. However, I am of the view that the indicator of a thin-walled ionization chamber brought in situ would be the best indicator of the effectiveness of an irradiation with X-rays and γ- rays. Whether this method is also applicable to β-rays depends on whether ioniza- tion chambers can be built with sufficiently thin walls to be inserted into the tissue. (Perhaps, for this investigation of intensity, water could be substituted for body tissue, as long as no bones are nearby.) (I could only judge this point with some certainty if the experimental research were made available to me, especially since, off the cuff, I cannot survey the mag- nitude of the technical difficulties.) V. How can one increase radiation absorption in tissue? What qualitative and quantitative importance should be attached to rays produced by incorporated gen- erators of secondary radiation? I believe that secondary-ray producers can only have a substantial effect if they can be brought into the tissue to be destroyed, or if the thickness of the tissue to be destroyed is so small that the secondary rays (β-rays of low velocity) can penetrate a large part of the tissue. On the whole, I do not believe that great improvement can be reached along this route. _______________ Publication denied in a letter to Lazarus.[5] E. 19. II 27. 439. From Sylvia Beach[1] [Paris,] 31 December 1926 [See documentary edition for English text.] [p. 3]