I. FANKUCHEN AND H. MARK 



If we now concentrate our attention on the immediate neiglibor- 

 hood of the incident beam in Figure 1, we find there is a distinct blacken- 

 ing of the film in this region. The intensity scattered at small angles 

 corresponds to large distances inside the investigated sample. If these 

 distances are arranged in some kind of periodic sequence, they produce, 

 according to Bragg's law, diffraction maxima the intensity and sharp- 

 ness of which depend upon the perfection of the periodic long-range 

 structure. If, therefore, the small-angle scattering consists of a number 

 of (more or less) well-defined lines, one concludes that something like a 

 "superlattice" exists in the crystalline domains of the sample. Figure 2 



shows a typical case of such a long- 

 range order in a suspension of 

 I tobacco mosaic virus (7). The 



very elongated rodlike particles 

 of this virus can be perfectly 

 oriented in their aqueous suspen- 

 sion even at such low concentra- 

 Fig. 2. — X-ray diaffram of tobacco ■ ^ ^rr-f , 



° . T, , , tion as 2 or 3%, and assume an 



mosaic virus. Beam normal to molec- 

 ular direction. order similar to that in a bundle 



of pencils: their axes are all 

 parallel and they maintain fixed average distances from each other. 

 If such a structure is irradiated with x-rays perpendicular to the 

 axis of orientation, patterns such as those in Figure 2 are obtained. 

 The sharp lines found on the equator of the diagram at small angles 

 indicate that there is a certain periodicity perpendicular to the axis of 

 orientation, which roughly corresponds, in the case of the dry virus, 

 to the average thickness of the rodlike parallelized virus particles. In 

 the case of tobacco mosaic virus, this thickness was found to be about 

 150 A. 



Comparatively sharp reflections at small angles on the meridian of 

 protein fiber x-ray diagrams have also been observed, indicating the 

 existence of a certain long-range periodicity along the axis of the paral- 

 lelized and aligned molecules. Astbury (3) has observed that |S- keratin 

 exhibits meridional small-angle reflections, which correspond to a 

 spacing of about 66 A.; he suggests they are either the third order of a 

 period of 198 A. or the tenth order of 660 A. Porcupine quill and 

 feather rachis indicate long periods of 198 and 95 A., respectively 

 (23). Beef tendon (5) exhibits about thirty orders of a fundamental 



446 



