INDUSTRIAL HYGIENE MICROSCOPY 



Fig. 5. Phase microscope. 



ors as the known. Both samples should be 

 examined at approximately the same tem- 

 peratm'e because the refractive index of a 

 liquid changes with a change in temperature. 

 For many liquids, the rate of change may 

 have a value of around 0.00045 for each 

 degree centigrade of variation. 



This temperature factor is of value in 

 some cases to obtain brighter dispersion 

 colors which aid in increasing the visibility 

 of small particle size dust. For example, if 

 the preparation containing quartz is exam- 

 ined in a warming stage in a liquid of 1.544 

 adjusted to that index at 25 degrees centi- 

 grade, increasing the temperature will de- 

 crease the index of the liquid. As a result, 

 quartz particles appearing blue with a trace 

 of red can be made to appear homogene- 

 ously red, orange or yellow^ dependent on 

 the temperature used. Consideration must 

 be made, that although the contrast on a 

 dark background is greater, identification by 

 dispersion colors other than blue with a 

 trace of red results in decreased accuracy. 

 However, this possible error is reduced pro- 

 vided a sample of known material is simul- 



taneously examined with the unknown by 

 means of comparison dispersion staining 

 microscope, comparing the change in color of 

 both samples as the temperature is in- 

 creased. 



Counting Dust Particles 



The number of dust particles in the air is 

 usually determined by first collecting the 

 dust in an impinger tube using water or al- 

 cohol as the collecting liquid. After collec- 

 ion, a 1-mm aliquot of the sample is placed 

 in a dust counting chamber such as a Sed- 

 wick-Rafter or Dunn cell and counted under 

 the light-field microscope employing a lOX 

 objective in combination w^ith a 7.5 X or 

 10 X eyepiece. Higher magnification oculars 

 can be used but the resulting magnification 

 should not exceed 1000 times the numerical 

 aperture of the objective. 



An objection to the use of the light-field 

 microscope is the fact that dust particles of 

 small size, having little or no color and close 

 to the index of the collecting liquid, will be 

 difficult or impossible to see. For this reason, 

 we have suggested (4) the use of the dark 

 contrast phase microscope (Fig. 5) for count- 

 ing dust particles which results in greater 

 accuracy and ease in counting. Fig. 6 is a 

 comparative photomicrograph of magnesium 

 fluoride dust in isopropyl alcohol as observed 

 by both light-field and phase microscopy. It 

 is rare that dusts of such low index and 

 proximity to the index of the collecting 

 liquid will be encountered in the usual work- 

 ing atmosphere. However, these photomicro- 

 raphs serve to illustrate the effectiveness of 

 the phase microscope for rendering visible 

 dust particles having a refractive index 

 differing only slightly from the refractive 

 index of the examination liquid. According 

 to Chamot and Mason (2), the use of the 

 phase microscope gives values to an addi- 

 tional decimal place beyond that obtained 

 by the usual Becke line method. 



A number of papers have been published 

 in the last few years suggesting the use of 



406 



