X-RAY MICROSCOPY 



fleeted by grains at the proper Bragg angle 

 with respect to the incident beams of given 

 wavelength. For an entirely unstrained 

 material there will appear magnified images 

 of these grains (the greatly inclined reflec- 

 tions produce streaked images as in Fig. lb 

 for commercial aluminum strip electro- 

 polished). When the strip is elongated 20 per 

 cent and again electropolished, these uniform 

 grain images break up into an intricate array 

 of fragments (0.005 mm in size) as shown 

 in Fig. Ic. Even 1 per cent elongation shows 

 easily apparent distortion of the internal 

 structure. 



Fig. 1. (a) Barrett diffraction microscope tech- 

 nique for enlarging grain images, (b) Patterns by 

 Barrett technique of polj'crystalhne akmiinum be- 

 fore application of tensile stress, showing enlarged 

 unrestrained grains, (c) Patterns by Barrett tech- 

 nique of polycrystalline aluminum after elonga- 

 tion, showing sensitive detection of strain. 



Mile. Cauchois has constructed a micro- 

 scope with a curved crystal of mica (2) . The 

 x-ray beam undergoes regular Bragg reflec- 

 tions from crystal planes, but these serve the 

 same purpose as curved mirrors, and an im- 

 age of a specimen, itself serving as the source 

 of x-rays or bathed by a beam of rays, may 

 be formed as indicated in Fig. 2 and en- 

 larged in accordance with the same prin- 

 ciples and equations as apply to light optics. 

 Guoy, von Hamos, and others mentioned in 

 Mile. Cauchois's paper anticipated the de- 

 velopment of optics with plane and bent 

 crystals. 



Fig. 3 shows the x-ray image spectrograph 

 devised by von Hamos, employing a curved 

 crystal which focuses secondary fluorescent 

 radiation to true monochromatic images of 

 the specimen in accordance with the equa- 

 tion 



2X 



- 1 



where 2X is the distance between corre- 

 sponding points in the sample to the image 

 in the film, R is the radius of the bent-crystal 

 surface, and n (order of reflection) X (wave 

 length) and d (crystal interplanor spacing) 

 in the Bragg equation. The intensity of the 

 image, which is an enlarged representation 

 of the distribution of the chemical elements 

 in the specimen whose secondary character- 

 istic rays are registered, is measured with a 

 microphotometer. This pemiits a true micro- 

 analysis of a very small specimen. For exam- 

 ple distinct zinc peaks are produced for only 

 0.2 7 of zinc in the area of the thin microtome 

 section of pancreas 200 /i thick, exposed to 



OB=p OB'=p' OF=f 



Fig. 2. The production of optical images from a bent crj^stal (Cauchois). 0, bent crystal; F, focal 

 point, AB, specimen; A'B' image. 



570 



