10 mm. The x-ray microdiffraction camera cannot be used for precision measurements, as all distances are small. In the region 2.5 to 3 A the error is estimated to ±0.02 A in the determination of interplanar spacings. The camera can be modified to include also long spacings, according to Bergmann and Fankuchen (5). As the collimator in the Chesley camera consists of a fine glass capillary, internal total reflexion of the x-rays in this capillary begins to interfere at spacings of about 60 A. Figure 8.19c shows the 43-A ring of a nerve specimen obtained in a modified Chesley camera. The equatorially oriented bands at higher spacings are not resolved.
There appear to be several possible ways in which the microdiffraction method might be refined for precise histochemical work. The development of the x-ray microscope (34) may provide a means for producing very fine x-ray beams. Thus, it may be possible to obtain diffraction from areas smaller than 1 μ square. It also seems important to make a precise study of the low angle diffraction from small areas in biological systems, and this may be achieved by using simple aperture collimation on a microscale in combination with fine-focus tubes (12,43).
Finally, it may be of advantage in many cases, e.g., in the determination of particle size from the diffuse low-angle scatter (29), to vary the type of radiation used. Thus, when A1 Kα is used instead of Cu Kα, the scattering power can be considerably increased, and the required thickness of the sample may he reduced.
The historadiographic methods which have already been developed and which have been discussed in this chapter permit certain chemical analyses of structures as small in order of magnitude as 10 μ3. The x-ray microscope now being developed [ Kirkpatricket al. (34)] may make it possible in the future to perform certain chemical analyses on submicroscopic particles. The x-ray microscope cannot compete with the electron microscope with regard to resolving power, but it has a very great advantage in that the images recorded can be related to simple absorption processes in the sample and can thus be interpreted chemically.
When performing a cytochemical analysis by x-ray absorption, as in all types of absorption measurements, many factors may interfere and give an incorrect result (28). Most of these factors lie in the sample itself, are inherent in the biological material, and are difficult to control. Since the specimens have to be introduced into vacuum, the drying effects have to be considered and investigated. The specimens have to be fixed and eventually sectioned with a microtome in order to render