156 MICROSCOPIC TECHNIQUES 



phy lies in its inability to reveal distribution in the finer structures, 

 and to this lack of resolving power must be added the further draw- 

 back that, quantitatively, only a rough approximation is possible. 

 However, there is the considerable advantage, inherent in the use of 

 radioactive elements regardless of whether a histological or gross 

 tissue study is involved, that very small quantities of an element 

 introduced into a biological system can be followed without reference 

 to, or interference from, the large stores normally present. The 

 amount of a radioactive element that can be detected is fortunately, 

 very minute. According to Hamilton (1941) a total of 2 X 10^ 

 (i particles, with an average energy of at least 150 Kev., are 

 required to strike each cm.^ of photosensitive surface to yield a 

 satisfactory image. 



Reviews dealing with radioautography have been presented by 

 Hamilton (1941-1942) and Simpson (1943), and important physical 

 data have been furnished in a review by Kurbatov and Pool ( 1943) . 



Preparation of Radioautographs* 



Both fresh-frozen and paraffin sections of tissue have been used 

 to obtain radioautographs. In general the paraffin sections give the 

 hest results since they can be cut thinner and are less subject to 

 distortion. It is essential that the sections be of uniform thickness 

 and free of wrinkles. There would be a particular advantage in the 

 use of freezing dehydration (page 3) for the preparation of the 

 paraffin-infiltrated tissue. The diffusion of the radioactive substances 

 during fixation and dehydration in solutions would be eliminated 

 and a more authentic "autograph" could be obtained. 



As examples of procedures which have been used the following 

 may be cited: Hamilton, Soley, and Eichorn (1940), in a study of 

 radioactive iodine in thyroid tissue, removed the paraffin from 3-5 /x 

 sections with xylol, dipped the slide containing the sections in dilute 

 celloidin, allowed it to dry, and obtained a celloidin film over the 

 sections about 1 fx. thick. The sensitive surface of the photographic 

 film was placed in contact with the celloidin surface. Harrison, 

 Thomas, and Hill ( 1944) in an investigation of the distribution of 



* See Bibliography Appendix, Refs. 20, 21, 22, 2S, and 30. 



