CHEMICAL MICROSCOPY 



birefriiigent. Interference figures, while im- 

 portant in optical crystallography, have been 

 very Httle used in niicroerystal tests. In 

 most cases it would be a waste of time to 

 look for them, as they could not be found. 

 On the other hand as soon as the analyst 

 learns to recognize the kinds of crystals on 

 which it may be possible to find them, they 

 become an added diagnostic characteristic 

 of value, and can often be obtained, even 

 using the 20 or 21 X objective, quite clearly 

 on surprisingly small crystals (e.g., down to 

 about 25 M diameter, in the case mentioned 

 in the previous article). Moreover, the sign 

 of the crystal can then usually be obtained, 

 as distinguished from the sign of elongation, 

 which may or may not be the same, or the 

 crystals may not be elongate. Some kinds of 

 crystals will give only indistinct figures, and 

 for test purposes it is not worth trying to 

 see them. 



A good example, but one seldom followed, 

 was set by the Behrens-Kley text in stating 

 actual sizes of microcrystals. No doubt 

 merely saying small, medium, or large is 

 often sufficient, and of course in a compari- 

 son one can instantly see whether the con- 

 trol crystals are of the same order of size as 

 the crystals obtained with the sample. 

 Moreover, one must often allow for change 

 of size with various factors; dilution or stir- 

 ring, for example, may cause diminution in 

 size. However, if an ocular micrometer 

 (kept in an extra ocular) has once been cali- 

 brated it certainly does not take long to use 

 it, and the size of fairly uniform crystals 

 under controlled conditions (i.e., in the test 

 as usually made) can be a valuable and 

 measured characteristic. Also the ratio of 

 length to breadth of oblongs, for example, is 

 then measured rather than estimated, and 

 similar proportions for other shapes. These 

 refinements may be reserved for important 

 tests, but ought not to be completely neg- 

 lected. 



Refractive phenomena have been seldom 



noted in these tests. Measurement of refrac- 

 tive indices can be used in a few cases with- 

 out invoking special procedures of filtration, 

 purification, recrystallization, etc.; e.g., 

 when a bircfringent acidic substance is pre- 

 cipitated from dilute NaOH solution with 

 HCl, and the drop then allowed to dry up. 

 However, even this involves more than sim- 

 ply observing the crystals in the test-drop 

 in which they form, the topic here. 



Observation with an ordinary microscope 

 which supplements the polarizing micro- 

 scope is often useful, chiefly as a matter of 

 convenience when it is troublesome to 

 change objectives on the polarizing micro- 

 scope because they are the type sliding on, 

 while those on the ordinary microscope are 

 on a turntable. Sometimes one wants to see 

 what the crystals look like in ordinary light; 

 the appearance of pleochroic or highly di- 

 chroic crystals is sometimes noteworthy. 



Darkfield observation may also be used. 

 Many kinds of crystals show up remarkably 

 well, but there is not the distinction between 

 crystals and non-crystals usually obtained 

 with polarized light and crossed nicols. The 

 role of darkfield therefore cannot be more 

 than secondary. In fact, no vital distinctions 

 (not seen otherwise) have been observed in 

 this way. At present it is not particularly 

 recommended, since to have a darkfield 

 ready for immediate use still another micro- 

 scope might be reciuired. With a phase micro- 

 scope both ordinary light and darkfield 

 effects may be observed, but phase micros- 

 copy has not proved of value in these crystal 

 examinations. 



There is one other mode of observation of 

 great advantage in a few cases, namely, 

 use of incident light. This cannot supplant 

 transmitted light, which is far more useful, 

 but can supplement it. A light should be set 

 up beside the microscope, which will throw 

 a beam down on the stage; it is then no 

 trouble at all to shut off the transmitted light 

 and turn on the incident light. Usually the 

 results are negative; that is, the crystals can 



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