INDLSTKIAL J1V(;IE.\E MICROSCOPY 



Fig. 4. Comparison eyepiece. 



refractive index liquids, the toxic dast such 

 as quartz could be obser\'ed in one color, 

 material not c^uartz in other colors of the 

 spectrum or white if far removed in refrac- 

 tive index. 



A variation in the impinger method of 

 collection ls to use the identifjang index 

 liquid as the collecting liquid in place of 

 water or alcohol. In this case, Uquids of less 

 volatility than st\Tene should be used. In- 

 dex Uquids cormnonh' emploj'ed are a mix- 

 ture of two hquids which may differ .slight h' 

 in vapor pressure. As a result, during the 

 procedure of collection of the .sample, there 

 may be greater evaporation of one of the 

 constituents resulting in a slight change in 

 refractive index. For accurate results, after 

 collection of the dust .sample, a drop of the 

 collecting hquid should be checked on a 

 refractometer and adjustment of its index 

 made if necessarv bv addition of either a 



small amomit of the lower or higher index 

 components. Using the identifWng hquid as 

 the collecting medium, evaporation on a 

 water bath is not neces.sary. The impinger 

 sample can be stirred with a rubber police- 

 man and a drop transferred to a shde for 

 microscopic examination. 



Quantitative results obtained should not 

 be based on the e.xamination of one sample. 

 Although di.spersion staining is probably the 

 simplest method of microscopic examination, 

 considerable experience is required in the 

 case of small particle size du.st. It has been 

 our experience that more reliable information 

 can be obtained by preliminary examination 

 of a known sample, such as quartz, previous 

 to e.xamination of the miknown, in order to 

 determine exact dispersion colors on rotation 

 of the cap analyzer. The unknown is exam- 

 ined in the same way to determine whether 

 particles present show the same shade of col- 



405 



