316 VOLUNTARY MUSCLE IN POLARIZED LIGHT. [BOOK I. 



Pjjebe. The property of double refraction in muscular 



Saviour of tissues has already been mentioned. Its discovery 

 muscle to po- W as made 1 in striated muscle; where also its conditions 

 larized light. ^ave been more fully observed than in the smooth 

 variety. A convenient apparatus for demonstrating double refraction 

 in microscopic objects consists of two Nicol's prisms, one the polar- 

 izer fixed between the illuminating mirror of the microscope and 

 the object stage, and the other called the analyser and capable of 

 rotating about the optical axis of the instrument interposed between 

 the ocular and the observer's eye. When the planes of polarization 

 of the two Nicols are at right angles the prisms are said to be 

 crossed, and the field of view is darkened ; when they coincide the 

 field is brightest. If, when the Nicols are crossed, a doubly refracting 

 body is interposed between them; if for example a plate of doubly 

 refracting crystal, cut parallel to its axis, is laid upon the stage of 

 the microscope; the analyser no longer blocks the rays, and the 

 field again becomes bright. The degree of brightness varies according 

 to the direction of the axis of the doubly refracting plate: it is 

 greatest when this axis makes an inclination of 45 with each 

 Nicol's plane ; and it is nil when it coincides with either of 

 these. If muscle, or any part of muscle, behaved like such a plate 

 of crystal, we should ascribe to it similar double-refracting properties. 

 A more beautiful way of demonstrating the optical properties of 

 muscular tissue is to interpose a very thin plate of doubly refracting 

 selenite or mica between the crossed Nicols. In this case, as in the 

 above experiment, light is transmitted or not through the analyser 

 according to the inclination of the axis of the plate; but the light is 

 not white, it is coloured. The particular colour depends upon the 

 thickness of the plate; and the most useful thickness is that which 

 gives a purple tint to the field with the proper inclination of the 

 axis. Supposing this to be attained, and supposing also that we have 

 that relation of the plate to the prisms which secures the highest 

 intensity or fulness of colour, we shall find that, as we rotate the 

 analysing Nicol, the intensity of the tint will diminish to its 

 vanishing point, at 45, beyond which the complementary tint will 

 appear, and increase to its maximum fulness at 90; and so alter- 

 nately through every quadrant. If now we place upon the mica 

 plate a doubly refracting body, its colour will be found to differ 

 from that of the field according to its doubly refracting character, its 

 thickness and the inclination of its axis to the crossed Nicol planes. 

 The advantage of the arrangement is that we may discriminate 

 between isotropous and doubly refracting bodies, not merely by 

 different intensity of light, but by more easily detected differences of 

 colour. 



By the aid of such appliances Brucke 2 wals able to determine 



1 C. Boeck (in 1839) ; reported in Arch. f. Anat. Physiol. u. wiss. Med. (J. Miiller), 

 1844, p. 1. 



8 Brucke, "Muskelfasern im polarisirten Lichte." Strieker's Handbuch, Chap. vi. 

 p. 170. 



