The aim of histo- and cytochemical studies is to identify and eventually to quantitate substances on a microscopic level. This necessitates isolating extremely small areas for analysis, preferably in the intact biological tissue. Physical methods have proven to be of value in this respect, in particular the radiant energy micro-absorption techniques (Fitzgerald & Engström, 1952).
The field of histochemistry is rapidly developing. In this summary, X-ray methods that have found use in histochemistry and histophysiology will be briefly discussed. Critical reviews (Glick, Engström, & Malmstrom, 1951; Fitzgerald & Engström, 1952) have recently been published.
The usual principle that has been adopted to obtain X-ray images of high definition is to place the object in close contact with a fine-grained photographic emulsion and make the exposure with X-rays as parallel as possible. The best emulsions, Eastman Kodak Spectroscopic Plate 548 or 649, Kodak Maximum Resolution Plate, Lippman emulsion (Gevaert), have a resolving power of about 1 μ.