POLARIZING MICROSCOPE. 121 



are two positions in which the field is quite dark ; that 

 is to say, the polarized ra} T s are entirely cut off. By ob- 

 serving now the relations of the prisms at these positions 

 of total darkness, it will be found that their planes of 

 polarization as shown by the way in which the prisms 

 are cut are at right angles to one another. In all inter- 

 mediate positions a greater or less amount of light is 

 enabled to traverse the analyzer. But if any object 

 which possesses the property of refracting light doubly 

 is placed upon the stage of the microscope, and examined, 

 and if then the field is made dark by turning the ana- 

 lyzer, it will lie found that the doubly refracting substance 

 remains bright, unless it happen so to lie that its optic 

 axis is parallel with the plane of polarization of either 

 Nicol. And if the object be a muscular fibre at rest, the 

 whole fibre will appear bright and doubly refracting, 

 whereas if it be in the state of contraction the bright 

 stripes only will allow the light to pass, the dark stripes 

 in this condition of the fibre being singly refracting. 



These observations are best made upon the living 

 muscle of the water-beetle by aid of the gas-chamber. 

 The portion of fibre under observation should be quite 

 free and not overlaid by other fibres. The change in the 

 optical condition of the fibres which ensues on contrac- 

 tion ma}', if due care and patience be exercised, in this 

 way be made out. The results arrived at by the exami- 

 nation of portions of the tissue which have been hardened 

 in alcohol, and mounted in glycerine or Canada balsam, 

 although more easily seen, are less trustworthy, since 

 the muscular fibres are liable to undergo considerable 

 changes after death, and under the action of re-agents. 



A pretty modification may be made by substituting a 

 thin piece of mica for the covering glass. This causes 

 the field of view to become tinted, the particular color 

 varying with the thickness of the mica, and the relative 

 position of its optic axis to those of the Nicols, and any 

 doubly refracting substance which is now examined 

 assumes the color which is complementary to that of the 

 field. The object of the revolving stages with which the 

 larger microscopes are generally fitted is to enable the 

 observer to modify the position of the optic axis of the 

 tissue which is being examined, with relation to those of 

 the Nicols ; and it also serves when the mica is used to 

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