The accuracy of the immersion method depends on the accuracy to which the oils are calibrated, whether the illumination is white or monochromatic light, and the quality of the polarizing microscope optics. For routine work, an accuracy of about +0.003 is possible when white light is used. The accuracy can be improved to about +0.001 if a sodium vapor lamp is used to provide monochromatic yellow light. By carefully controlling all various factors involved can improve the accuracy obtained but it is not necessary for most mineral identification purposes. The variation between different samples of the same mineral is often greater than the errors involved in routine measurements.
The increments of the commercially available immersion oils are usually available on 0.002, 0.004, or 0.005. By mixing two of the oils in various proportions. Intermediate indices can be obtained. The simplest yet moderately accurate method is to place equal-sized drops of the different oils on a slide or watch glass and then mix. Another method that can be employed is by using a more precise method of measuring the amounts of the liquids to be mixed. The index of the mixed oil should be measured with a refractometer if great accuracy is required. Refractometer is an instrument used to measure refractive indices.
The contamination of the index oil is one source of inaccuracy in measuring index of refractions of minerals. Index oil contamination commonly occurs by touching a dropper to a dirty slide and interchanging the droppers or bottle lids. Also, if the containers of index oil are left open, the index of refraction of oil may be altered. This is so because some of the constituents of the oil are allowed to evaporate.
The indices of the oils also vary as a function of the temperature. Most oils are calibrated at normal room temperature of about 20degree Celsius. A correction factor should be applied if the temperature varies from the normal room temperature. The temperature at which the oil should be calibrated and the temperature correction of the oil is printed on the bottle of the commercially prepared immersion oils. The index of refraction decreases with the increasing temperature. The correction factor of most immersion oils is usually about –0.0004 per degree Celsius. So it is just a few degrees variation from the normal room temperature of 20 degree Celsius and is not significant for routine works.
The temperature variation of the immersion oil can be used to advantage only if a heating stage is available. A sample is prepared so that the index of oil at normal room temperature of 20 degree Celsius is higher than that of the mineral. The index of refraction of the oil is reduced to match that of the mineral and this can be done heating the sample on the heating stage if available. The index of refraction of the mineral can be calculated using the correction factor. This is so because the index of refraction of the mineral now varies little as a function of temperature. The calculation however can be done only if the temperature at which the match is obtained and temperature correction factor is known. This is best applied to high degree of accuracy required and to situations where monochromatic light is used.
