140 MICROSCOPIC TECHNIQUES 



The raetluxl for tlie estimation of the j)roportion of reduced silver 

 is based on counting the silver grains in the photographic emulsion 

 on the film, following the procedure of Glinther and Wilcke (1926i. 

 The method is only adapted to low densities (upper limit d = 0.27) . 

 A microscopic enlargement of 600 X is used for coarse-grained film 

 and 1350X (immersion objective) for fine-grained film. The counts 

 are facilitated by the use of a netted ocular having 100 squares. 

 The number of grains in an unexposed film area is determined in a 

 region immediately adjacent to that exposed. 



In a study of the properties of photographic emulsions, Engstrom 

 (1946, pages 65-72) pointed out that it is necessary to obtain a 

 curve of the relationship of photographic density to radiation 

 intensity for every wavelength, emulsion, and set of development 

 conditions in order to arrive at a suitable working arrangement. 

 Engstrom investigated the properties of Agfa, Laue, and Printon 

 films and Ilford High Resolution plates and presented curves of both 

 density and number of grains as functions of intensity. 



Nomogram 



Engstrom ( 1946) has published a nomogram for calculating the 

 analytical results according to the equation shown in Figure 28: 



"In this nomogram are included the most important elements, and the analj^- 

 sis assumes the employment of the K-absorption edge. The wavelengths for the 

 analysis lines used are seen [Table III]. The two outer pillars in the nomo- 

 gram indicate the quotient between the intensity of the transmitted and incident 

 roentgen radiation in the two wavelengths Xi and X2. The amount sought for 

 (A') of the respective elements is marked out on the vertical lines in the 

 centre. The figures after the respective elements indicate the amount of the 

 element in ciuestion at the end of the scale, e.g., 30 Zn 100 /ug./mm.- The 

 following example shows how it is employed: In determinations of nitrogen, 

 it is, e.g., found that ii//i is 0.06 and that i^Hi is also 0.06. The straight line 

 which joins these two points on the outer pillars cuts the ciu-\e for nitrogen 

 at the point A'. The end point on the nitrogen scale is 1.5 /xg.N/mm.^, and the 

 scale is divided up into 15 parts, from which it appears that the amoimt of 

 nitrogen sought for is 0.49 /xg./mm.^" 



E. MICROINCINERATION 



Microincineration is a valuable technique for the faithful repre- 

 sentation of the total mineral distribution in tissue sections. In its 

 present state of development, its reliability is evidenced by the fact 



