X-RAY DIFFRACTION 



part of the pattern. It is therefore customary to stop the primary beam 

 with a small lead plate, the shadow of which can be seen in the center 

 of Figure 1. In addition to these spots, one observes a fairly distinct 

 accumulation of scattered intensity in the immediate neighborhood of 

 the incident beam and a faint, but not negligible, diffuse background 

 upon which the diffraction spots are superimposed. 



Theoretical considerations (see 9,24) and extensive experience 

 with x-ray diagrams of many inorganic and organic materials indicate 



Fig. 1. — X-ray diagram of commercial silk fibroin 

 (silkworm). Beam normal to fiber axis (24). 



that, under favorable conditions, conclusions of the following character 

 can be drawn from the analysis of such patterns. 



Mere qualitative inspection permits the conclusion, on the basis of 

 the existence of distinct spots, that the sample investigated contains 

 geometrically organized areas inside of which long-chain molecules of 

 the silk protein are localized in a strictly periodic manner, like the indi- 

 vidual bristles in a paint brush or the single pencils in a package of 

 pencils. The presence of distinct spots instead of rings indicates that 

 these highly ordered domains all have one of their axes parallel (or 

 very nearly parallel) to the common axis of the investigated fibers, 



441 



