BEHAVIOUR OF MUSCULAR FIBRES IN POLARISED LIGHT. 243 



one another or crossed, it presents a beautiful purple colour ; 

 the muscular fibres then appear blue or yellow, according to 

 their inclination. Amongst the different purple tints which can 

 be obtained, that is the best which first appears in increasing 

 divergence of the rays with crossed prisms, and which corre- 

 sponds to the purple which is exhibited by Newton's colour 

 glass in reflected light at the limit between the first and the 

 second system of rings. It furnishes in particular the most 

 sensitive field; that is to say, small differences in the divergence 

 of the rays occasioned by doubly refracting bodies lying upon 

 the plate are rendered manifest by relatively great changes of 

 colour. From preliminary investigation with the polarising 

 microscope it is easy to discover, out of a series of gypsum or 

 mica plates of various thickness, those that are best adapted 

 for this purpose, attention being paid not only to the colours 

 themselves, but to the amount of change of colour occasioned 

 by small accidental variations in the thickness of the sections. 

 If the little plates which are used for preserving the prepara- 

 tion contain air between their lamellae, which collects into bub- 

 bles in the preliminary immersion in oil of turpentine, this 

 can be expelled by boiling in turpentine, and allowing it to 

 remain in it till cold. It may then be transferred to the 

 balsam or varnish, with which it and the muscular fibres lying 

 upon it are to be enclosed. 



