368 POLARISATION. 



intention of discovering the relative excentricity of the effective 

 ellipses of elasticity from the colours which they produce. If, for 

 instance, a membrane 6 mic. thick gives in the surface view, 

 with a selenite plate red I., the acceleration colour blue II., whilst 

 a transverse section produces the same effect with 4 mic., it does not 

 follow that the ellipse of elasticity of the section possesses a rela- 

 tively greater, or in general a greater, excentricity than that of the 

 surface view ; for with the latter the more effective dense layers 

 form only a fraction of the total thickness, whereas in the section 

 they run continuously from one sectional surface to the other. A 

 comparison in the given case is therefore only admissible between 

 the different tangential directions, as they become effective in 

 transverse, longitudinal, or any oblique sections, if they pass 

 through a radius. 



It is manifest that surface views of membranes, whose layers 

 produce colours of opposite character in transverse section, do not 

 admit of interpretation according to the rules we have laid down, 

 when more than two such layers are present. 



VI. 



COLLECTION OF EXAMPLES. 



ALL structures hitherto investigated agree in that one of the 

 three axes of elasticity is situated radially. The two axes lying in 

 the tangential plane exhibit the most variable positions ; occasionally 

 they correspond to the longitudinal and latitudinal directions of the 

 cells or fibres, or they may cut these directions at different angles. 

 We will hereafter denote by left-handed rotation and right-handed 

 rotation the positions in which the longer of the two tangential 

 axes lies with reference to the direction of a left-handed or right- 

 handed spiral (in accordance with botanical terminology). 1 The 

 angle which the spiral coils form with the longitudinal axis we will 

 call </>. As regards the position of the ellipses in the transverse 



1 A common screw has, according to botanical terminology, a left-handed 

 thread, whilst in mechanics it is termed right-handed. 



