INDLSTIU A!. IIY(;iENE iVIICHOSCOPY 



emitting from single refracting particles is lower index. Complete disappearance of the 

 rejected by the analyzer located above the particle being examined, indicates that it has 

 specimen. Quartz in the same preparation an index of 1.544 for yellow light, the index 

 appears bright. This is explained by the fact for the ordinary ray of crystalline quartz, 

 that double refracting particles divide the The analyzer is now reinserted and the 

 ray of light from the polarizer into two rays stage rotated to the second extinction posi- 

 (ordinary and extraordinary ray). As in the tion and estimation made on the same par- 

 case of the opal, the ordinary ray is rejected tide as to whether the refractive index for 

 bj'' the analyzer but the extraordinary ray, the other component vibration is higher than 

 vibrating at right angles to the ordinary 1.544. If examination by the Becke line 

 ray, is transmitted through the analyzer. As method proves this to be true, the above 

 a result, the quartz particles appear bright described procedure, as used for the 1.544 

 on a dark background. index, is now repeated using a liquid of 



This preliminary microscopic examination 1.553. Disappearance in this liquid indicates 

 serves to classify the unknown as to whether that the particle in question has a second 

 it belongs to the isotropic or anisotropic index of 1.553 for the extraordinary ray and 

 group of chemicals and minerals. Additional can thus be considered crystalline cjuartz. 

 optical properties such as refractive index Additional identifymg procedures such as 

 or indices are necessary for identification. In examination of interference figure and de- 

 view of the possibility that the bright aniso- termination of optic sign used by the petrog- 

 tropic particles (Fig. 2) might be crystalline rapher for mineral specimens are difficult or 

 quartz, a Handbook of Chemistry or Text- not possible on small particle size dusts, 

 book of Mineralogy is consulted. Informa- A simplified procedure for the identifica- 

 tion obtained indicates that quartz has two tion of dust particles is to examine the sam- 

 refractive indices, 1.544 for the ordinary ray pie with a dispersion staining dark-field mi- 

 and 1.553 for the extraordinary ray. croscope. This method (1) of illumination is 



Determination as to whether the unknown obtained by the addition of the accessories 

 has these refractive indices can be made by shown in Fig. 3 to a laboratory type micro- 

 use of a petrographic microscope illuminated scope. For small particle size dust, a combi- 

 with a sodium light source. The sample is nation of a 20 X (10.25mm) 0.40 N.A. ob- 

 mounted under a cover glass in a liquid of jective with a 20X hyperplane eyepiece is 

 refractive index 1.544 for yellow light, and suggested. The condenser employed is a 1.40 

 examined with the analyzer and polarizer of N.A. achromatic condenser but with the top 

 the microscope in a crossed position. The element removed to give an N.A. of 0.59. 

 stage of the microscope is rotated until a The diameter of the dark-field stop used 

 particle being observed appears dark. The below the condenser is very critical, a diam- 

 analyzer is then removed and the index of eter of 17 mm usually giving good results, 

 the particle with respect to the liquid is de- Use of a cap analyzer over the eyepiece is of 

 termined by the usual Becke line procedure, value for the identification of anisotropic 

 Using this method, a halo of bright light dusts such as quartz. 



may be observed moving into or away from As in the usual petrographic method de- 

 the particle as the focus of the microscope scribed above, the sample is placed in a re- 

 is slightly changed from exact focus. If the fractive index liquid which in the case of 

 halo moves into the grain on the up focus, quartz would be 1.544 for yellow light. In 

 the particle has a higher index than the place of a sodium lamp, a 6 volt — ^108 watt 

 liquid; movement of the halo away from the ribbon filament lamp is used for illumination, 

 particle into the index liquid signifies a Using this dark-field method, yellow light 



402 



