INDUSTKIAL IIYGIKNE IVIICKOSCOPY 





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Fig. 7A. Diatomaceous earth particles on a 

 Millipore type AA filter. Cleared in a liquid of 

 refractive index 1.507. Light-field illumination. 



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of litjuids vary with temperature. Slight dif- 

 ferences are not important but use of a 1.507 

 clearing liquid corrected for that index at 

 25°C and used at a much lower or higher 

 temperature may result in some objection- 

 able visibility of the filter. 



Deterniiiiatioii of Particle Size 



Determination of the particle size of the 

 dust at the approximate breathing area of 

 the worker can be made by examination of 

 the impinger sample collected for the purpose 

 of making the dust count. If a hemacytom- 

 eter was used as the counting chamber, some 

 idea of the particle size can be obtained by 

 comparison with the ruled areas of known 

 size. Increased accuracy is obtained by use 

 of a micrometer disc in the eyepiece or a 

 Filar Micrometer eyepiece (Fig. 9). Before 

 use, the divisions of the micrometer must be 

 calibrated by use of a stage micrometer hav- 

 ing a scale of known area, the smallest divi- 

 sion being .01 mm (10 microns). Calibration 

 of the eyepiece disc is made by first focusing 

 on the stage micrometer. The stage microm- 

 eter is then moved to a point so that one line 

 of it coincides with a line to left center of 

 the eyepiece scale. Count is then made 



Fig. 7B. Exactlj^ the same area as figure 7A. 

 Phase illumination. 



satisfactory. Another possibility if this . 

 1.507 liquid is not available, is isoamyl 

 salicylate. It is a colorless liquid with pleas- 

 ant odor, has no solvent effect on the filter 

 and its refractive index is usually close 

 enough to 1.507 at 25°C as to render the r»W5f«^4 

 filter quite transparent. In using these lici- 

 uids, consideration must be made as previ- 

 ously mentioned, that the refractive index 



mA 



Fig. 8. 



Diatomaceous earth particles on a 

 Millipore type AA filter. Cleared in a liquid of 

 refractive index 1.515. Phase illumination. 



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