Sec. 14.2] THE RADIOAUTOGRAPII 383 



distribution of active material, but microscopic examination is more difficult 

 A frame designed to fix the slide or cover slip with respect to the film is of 

 some value for this purpose. The film and slide are fixed in their respective 

 frames and held together by alignment pins or angles during exposure. After 

 separate development and staining they may again be aligned and the tissue 

 slide fixed to the film in the original position. Both components may then 

 be removed from their frames. This procedure will yield a fairly satisfactory 

 preparation for microscopic examination under low power and, depending 

 on the thickness of the tissue preparation, perhaps under high power. The 

 use of the oil-immersion objective, which is necessary for cellular localization 

 of the radiation source, is impossible with this type of preparation. A 

 microscope comparator may be used whereby tissue section and film may be 

 aligned by a reference mark, such as a spot shadow of a small piece of gold 

 leaf, and picture and tissue examined simultaneously. Alignment in this 

 instance is not accurate enough to yield definite information concerning 

 cellular origin of radiation. 



The problems of realignment are solved by either of the last two methods 

 (2 or 3) listed above. In the second method, the section is placed on a slide 

 or cover slip, stained or not depending on the radioactive material being 

 studied, and covered with emulsion. If the section is stained prior to 

 exposure, it may be held on the slide by a very thin coating of collodion. 

 Liquid emulsion, prepared by formulas given in the literature or melted from 

 a plate or film, is then spread over the tissue. Preparation and handling of 

 liquid emulsions is an art, and great care must be taken to avoid excessive 

 grain size and high background fog. In addition, it is often difficult to obtain 

 a uniform and thin film necessary for preparation of good radioautographs. 

 However, a more satisfactory procedure is the use of an emulsion obtainable 

 in the form of stripping film, which is prepared so that the emulsion may be 

 removed from its backing and cemented over the section. After the proper 

 exposure and development of the film, the section is stained if this has not 

 already been done. Here and in the procedure outlined below, the usual 

 histological stains are of limited use because of the strong affinity of the 

 gelatin of the emulsion for the dyes. A completely satisfactory staining 

 technique that gives excellent tissue differentiation and does not stain the 

 emulsion has not yet been described. Another drawback in using liquid 

 emulsion or stripping film is distortion of the emulsion through handling. 

 This distortion is, of course, most serious in the case of liquid emulsions but 

 also occurs to some extent with the stripping film, which frequently during the 

 course of developing and staining becomes detached from the supporting 

 slide. Although separation may be unavoidable, the radioautograph is 

 still of value if developing and staining are finished carefully and the film 

 with section attached is dried between blotting papers. 



