A simple diffractometer is described, in which monochromatic light is focused on a pinhole, rendered parallel, and passed through a film of red cells or other objects the size of which is sought. The diffraction patterns are photographed on special plates, and the positions of the first minimum and of the first maximum are subsequently found by the use of a simplified microphotometer. The method gives substantially the same results for red cell radius and (calculated) volume as do other standard methods.

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