INSECT CUTICLE RICHARDS AND PIPA 



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in that plane. If pieces are cut or broken from such thin sheets and 

 examined on edge between crossed Nicols they are distinctly bire- 

 fringent, and both amplitude and path length are readily measured. 

 But, since orientation is random in the one plane, the measured ex- 

 tinction angle is only half maximum. Accordingly, one uses ^ = 2mAsin 

 2 B for the calculation. The necessary difference in calculation can 

 be illustrated by a diagram. 



In tendons purified for chitin one has the situation diagrammed in 

 figure I A. Here regularly parallel micelles and microfibers are 

 readily oriented perpendicular to the beam of polarized light. A 

 maximum effect is then obtained since the optic axis is parallel to 

 the long axis of the fibers. In the thin sheets of arthropodin one 

 has the situation diagrammed in figure i B. Here elongated molecules 



A B C 



Fig. I. — Optic axes and observational directions for tendons (A) versus thin 



sheets (B, C). 



are all in one plane, but randomly oriented in all directions in that 

 plane. Such a sheet is isotropic in surface view. When such a sheet 

 is examined on edge, some molecules are being seen parallel to the 

 light beam (and hence along their optic axis), some perpendicular 

 to the beam, and others at all intermediate angles. The average value 

 will be equal to that of regularly parallel micelles oriented at 45° 

 to the light beam, as diagrammed in figure i C. Hence a correction 

 factor of 2 must be added to the usual equation for calculation of 

 magnitude of birefringence. 



The refractive index of arthropodin was determined by the Becke 

 line technique following immersion of drawn fibers in mixtures of a 

 clear light mineral oil and a-bromonaphthalene. The determination 

 was checked using pure chemicals (bromobenzene, o-toluidine, nitro- 

 benzene, etc.). Dry and dehydrated tendons were examined in the 

 same media. Hydrated tendons were examined in glycerine and in 

 a saturated solution of KI (temp. 26-28°C.). 



