138 



MICROSCOPIC TECHNIQUES 



The radiation enters through the adjustable slit (B) and falls on 

 the crystal (C), whose angle may be adjusted by the micrometer 

 screw (D). The monochromatized beam from the crystal passes 

 through the tissue section in holder E. The holder can be moved in 

 relation to the photographic film placed behind the tissue by means 

 of the micrometer screw (/) . The film carriage can be adjusted along 

 scale F at a chosen distance from C. The angle of the tissue holder 

 can be set on scale G. The spectrograph whose dimensions are 

 10 X 20 X 6 cm. is evacuated through H. The lid is sealed to the 

 chamber with rubber and clamped tight. The films used were 

 12 X 12 mm. and the crystals employed were gypsum {d = 7578 



Fig. 27. Mounting of a preparation. 

 .A is a part of the preparation holder. 

 B is the preparation itself. C is a cross 

 of Wollaston wires (platinum) used 

 to obtain points of reference. 



From Engstrom (1946) 



X.U.) and mica (d = 9930 X.U.). The tissue section is mounted 

 over a hole (0.2-2 mm.) in a sheet of metal, Figure 27. The distance 

 between tissue and film is 1.5-2 mm. 



4. Measurement of Density of Photographic Film 



The intensity of the radiation is measured by the degree of the 

 blackening of the photographic film. The blackening may be deter- 

 mined by photometric estimation of the proportion of visible light 

 absorbed by the film or by measurement of the proportion of the 

 silver that has been reduced. 



The photometric estimation has been effected by two methods in 

 Engstrom's (1946, pages 60-64) work. One procedure utilized a 

 self-recording microphotometer (Siegbahn type) with a thermo- 

 element and Moll's microgalvanometer, and the other employed the 

 Caspersson photoelectric apparatus for the determination of light 



