COMMERCIAL LINT 139 



mention from our point of view, being the result of acci- 

 dental episodes which happen after the boll has opened, 

 or of insect attacks on the boll. 



Lustre is somewhat peculiar, It would seem to have 

 been insufficiently analyzed by investigators, and in the 

 first place it is probably almost synonymous 

 with twist; if all the hairs in a sample are 

 well and evenly twisted, there will be an infinite number 

 of convex surfaces, each reflecting a spot of light. In 

 addition to this there is refraction of light, which may be 

 seen by holding a well-twisted fibre against a dull back- 

 ground with a good north light well overhead. The fibre 

 then shows slight diffraction colourings; this undoubtedly 

 has considerable influence on the appearance of the 

 sample, through the reflection of light back from the 

 concave surfaces inside the fibre; it would necessarily 

 involve the translucency of the fibre wall, since any 

 opacity of the wall would obliterate it. Such opacity 

 might result from prolonged exposure of the seed -cotton 

 on the plants, or from irregular deposition of the thicken- 

 ing layers of the wall, in so far as single fibres are con- 

 cerned, while irregularities from fibre to fibre would have 

 a similar effect in a bulk sample of lint. In former dis- 

 cussions of lustre the major importance has been attrib- 

 uted to the cuticular skin of the fibre, and co variations 

 in its reflection of light; but cuticle is one of the last 

 plant tissues to be affected by ill-treatment, and it 

 seems more than probable that the causes of lustre 

 changes and variations lie behind the cuticle. 

 One point may hero be mentioned which the author has 



