INDUSTRIAL HYGIENE MICROSCOPY 



Fig. 3. Microscope accessories for dispersion staining. 



for which the quartz and Hquid are equal m 

 refractive index will pass straight through 

 the particle oblique to the optic axis of the 

 microscope and thus not enter the objective. 

 Blue and red light for which the quartz is 

 not equal in refractive index are refracted 

 at the particle-liquid interface to a degree as 

 to enter the objective. Due to the greater 

 bending of blue light as compared to red, the 

 particle when equal to the liquid for yellow 

 light appears largely blue with a trace of red. 

 If the small amount of red is somewhat diffi- 

 cult to observe, increased visibihty can be 

 obtained by very slightly decentering the 

 dark-field stop. 



Success with this method, especially in the 

 case of small particle size dust, is based on 

 the use of index liquids of much greater dis- 

 persion than the sample examined. The 

 greater the difference in dispersion between 

 the sample and the liquid, the more brilliant 

 the colors. For the identification of quartz, 

 styrene stabilized with ter t-huty\ catechol 

 has proved to be of value. It has an unusu- 

 ally high dispersion for its refractive index 

 of approximately 1.544. As examined in this 

 liquid with the dispersion staining dark-field 



microscope, using a cap analyzer over the 

 20 X hyperplane ej^epiece, quartz particles 

 oriented for the ordinary ray, 1.544 appear 

 largely blue with a trace of red. Particles 

 oriented for the extraordinary ray, 1.553 

 appear blue with a large amount of red or are 

 homogenously red in the case of very small 

 particle size. Rotation of the cap analyzer 

 90 degrees results in particles oriented for 

 the ordinary ray (appearing blue with a 

 small amount of red) to change in color to 

 blue with a large amount of red, or all red. 

 Particles oriented for the extraordinary ray, 

 change in color to those significant for the 

 ordinary ray. 



Additional dispersion colors can be ob- 

 served which are of value for identification. 

 In place of a 1.544 index Hquid, a 1.553 Hquid 

 equal to the extraordinary ray of quartz can 

 be used and notation made as to the change 

 in color on rotation of the cap anah'zer. 

 Dispersion colors can be observed indicating 

 whether the dust sample is above or below 

 the index liquid in refractive index. A pure 

 blue with no trace of red is obtained when 

 the particle is slightly lower in index than 

 the liquid; light blue signifies a still lower 



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