COLORED POLARIZATION. 
87 
112. IIerai>atBiite or Artificial Tourii(ialiiac§. These 
are crystals of disulphate of iodine and quinine^ and when of 
sufficient size can be used both as polarizers and analyzers. 
They are called Herapathite from the name of their discoverer, 
Dr. Herapath, but large crystals have not yet been produced 
in sufficient abundance to allow of their general use. 
113. Valite of the Polariscope isi Micro§copic iaivesti- 
gatioifi§. When the polariscope is attached to the microscope, 
the polarizer being below the stage and the analyzer above it, 
any object can be subjected to examination with polarized 
light. Objects having parts of their structure more dense than 
others, will present greater contrast of light and shade than by 
ordinary light, and thus the most delicate structures, as capil- 
lary blood-vessels, nerves, cell-walls, &c., will be well defined 
where they could not be otherwise distinguished. Sections of 
horn, teeth, bones, quills, shells, and many vegetable tissues, 
exhibit their delicate structure under the influence of polarized 
light. Revolving the polarizer causes the relative brightness of 
difierent structures to vary, and is essential in developing the 
greatest effect of polarized light in distinguishing delicate 
structures. Those portions of an object, possessing the power 
of partial or complete polarization, act like particles or veins of 
tourmaline to obstruct the passage of polarized light in certain 
positions, while the other parts of the object appear as lumin- 
ous as by ordinary light. So, also, when the position of the 
polarizer and analyzer is such as to cut off all the light from 
ordinary objects, the delicate structures that possess the property 
of polarizing light, depolarize the light already polarized and 
allow it to be transmitted, showing points or veins brilliantly 
illuminated amid other parts of the object which appear dark. 
It should therefore be laid down as a rule in microscopic in- 
vestigation, says Prof. Queckett, “ That every new variety of 
tissue should be subjected to the action of polarized light.” 
114. Colored Polarization. All substances, whether ani- 
mal, vegetable, or mineral, which, by the unequal arrangement 
of their particles possess the property of double refraction, 
when placed between the polarizing and analyzing prisms 
exhibit colors^ varying according to the otherwise unapprecia- 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
