Transmitted Brightfield, Closed SubStage Condenser Iris, 20X Objective
Dispersion Staining is an optical staining technique created by differences in the dispersion of the refractive indices for
a particle and the liquid in which it is mounted. Becke` Line dispersion staining is one of the five methods of dispersion
staining. It is used primarily as a screening technique or for large particles. For smaller particles or when looking for
a specific mineral the other methods of dispersion stain generally work better.
This approach works best with a mounting medium that has a steep dispersion curve. Most liquids with refractive indices
above 1.60 meet that requirement. There are "high dispersion" liquids sold commercially designed specifically for
dispersion staining. These sets normally start at a refractive index of 1.500 and go up to about 1.700. The particles of
interest are mounted in one of these liquids that matches the refractive index of the particles at some visible wavelength.
High dispersion liquids can also be made by mixing cinnamic aldehyde (R.I. about 1.62) with triethyl phosphate (R.I. 1.406),
or methylene Iodide (R.I. 1.737). A less expensive set of high dispersion liquids can be made with cinnamon oil, also
called cassia oil (R.I. about 1.60) and clove oil (R.I. about 1.53) or caster oil (R.I. about 1.48). These oils can
generally be purchased at any local drugstore. When liquids are mixed it is good to test them against standard glasses or
minerals on a regular basis. The commercial refractive index liquids are designed for long term stability.
The particles must be mounted under a coverslip to optimize the effects and minimize in interference cause by any optical
anomaly in an unmounted specimen.
Begin by establishing Koehler Illumination and then close down the sub-stage condenser iris. Defocusing the particle brings
the colored Becke` Lines into view.
The advantage of this technique is that no accessories to a standard microscope are required for its use. A disadvantage of
this technique is that the particle must be move from its optimal focal position to see the effect. In a field cluttered
with particles it can be difficult to be certain of the particle producing the effect. Image resolution is sacrificed and
depth of field is increased by closing down the sub-stage condenser iris. This further complicates the isolation of one
particle in field full of particles.