Polarizing Microscope in the Examination of Organic Bodies. 445 



me that the intensity of the light can hardly be too great; hence 



1 took especial care, in the selection of the apparatus, to ensure 

 obtaining as much light as possible. 



For this reason, and to obtain colourless light, I avoided 

 tourmalines or Herapathite, and made use of Nicol's prisms. 



In regard to the polarizing apparatus itself, it is advantageous 

 to use, as the polarizer, a Nicol's prism of tolerably large dimen- 

 sions, through which a broad beam of polarized light may pass 

 to the objects. The illumination of the object, however, is not 

 brighter with a large than with a small Nicol's prism; for when 

 the polarized light, after emerging from the prism, is condensed 

 to a luminous image upon the object by the condenser to be 

 mentioned presently, this is equally bright, whether the prism 

 be large or small ; but the size of the image depends upon the 

 size of the transverse section of the Nicol's prism used : hence, 

 when this is too small, observation is rendered difficult by a 

 small portion only of the object, lying in the centre of the field, 

 being illuminated. The Nicol's prism need not, however, be of 

 unusually large dimensions : a diameter of from 10 to 12 lines 

 answers every purpose. As, however, a prism of this size is 



2 inches long, care must be taken that there is sufficient room 

 between the mirror and the stage to admit both the prism and 

 the condensers. Hence this space should never be less than 

 4 inches. 



As regards the position of the polarizing Nicol's prism, the 

 question arises whether the relative direction of its oblique ter- 

 minal faces to the plane of the mirror is a matter of indifference, 

 or whether the horizontal diameter of its rhombic terminal faces 

 should hold a definite position to the horizontal axis of rotation of 

 the mirror. For, when all the lenses are removed from the micro- 

 scope, on placing a Nicol's prism upon the eye-piece, and turning 

 the mirror to the light, it will be found, on rotating the prism, that 

 the light reflected from the mirror into the microscope is polar- 

 ized to a considerable extent, and that a marked darkening is 

 produced when the transverse diameter of the rhombic terminal 

 face of the prism is parallel to the horizontal axis of rotation of the 

 mirror, and thus also to the window through which the light pro- 

 ceeds. Hence, when the polarizing prism is fixed in this posi- 

 tion under the object, less light will be transmitted to the latter 

 than when the prism is rotated 90° and retained in this position 

 during the observation. This circumstance is in general of no 

 great importance; for with the bright illumination which must 

 be provided under all circumstances, the loss of light caused by 

 a less favourable position of the polarizer is of far less importance 

 in regard to the brightness of the image than might at first be 

 supposed. 



