348 POLARISATION. 



In this synopsis only the first combination series in which 

 each plate separately gives less than white I., the combination 

 for 6 = therefore giving not quite red I. admits of the recog- 

 nition of a certain conformity to law. In this case the colour 

 of the orthogonal consecutive position, like the diagonal position 

 of acceleration, always remains above the red of the selenite 

 plate, the colours of the alternative and retardation positions,. 

 however, always below it ; " dark red " denoting the red which 

 forms the passage to violet II., " light red " that to red-orange. 1 



The second combination series is obtained by pairs of plates 

 of which each one separately gives white I., or a higher colour. 

 The colours of the different positions do not here admit of inter- 

 pretation, since they lack a definite acceleration or retardation 

 character. 



Let us now reverse the problem, and ask ourselves what data 

 the relations we have developed afford us for determining from 

 known polarisation colours the unknown ellipses of elasticity of 

 two superposed plates. First, it is clear that the colours of 

 the maximum acceleration or retardation, which are observed 

 on rotation upon a selenite plate, always correspond to the two- 

 positions which we have denoted by the diagonal position of 

 acceleration and retardation, and that they consequently disclose 

 to us the position of the acute angle e, made by the major axe& 

 of the ellipses of elasticity. The bi-sectional line of this angle is 

 thence determined. 



Secondly, observation decides whether the colours of the two- 

 orthogonal positions are sufficiently distinctly outlined that the 

 consecutive or alternative position may be recognized as such. 

 Where this is the case, the position of the lower and of the upper 

 ellipse of elasticity is determined in the sense explained above ,- 

 where it is not the case, the investigation can, as a rule, be 

 repeated with smaller objects of the same kind which give rise to- 

 a considerably stronger colour. With most organised structures- 

 (membranes, fibres, prosenchymatous cells, &c.), which act as a pair 

 of plates, we can thus succeed in producing a change of colour 

 which unmistakably belongs to the first combination series of 

 the above synopsis, and which therefore admits of an accurate 

 interpretation. 



i Cf. Nageli: " Die Anwendung des Polarisationsmikroskops," Beitrage III. 

 p. 98. 



