Xylem E-Line Gemological Refractometer - Manual (Page 4)

Operating Instructions
Method of Operation (Singly Refractive)
1. Place the instrument on a suitable table. Open the upper cover. Drop a few drops of
contact liquid (see table over page) onto the surface of the prism using a thin glass bar or soft
wooden stick. (CAUTION: Contact Liquid - Read Material Safety Data Sheet).
2. Gently place the polished surface of gemstone on the prism, depressing it lightly to make sure
contact liquid is distributed evenly between the gemstone and measuring prism.
3. Close the upper cover.
4. Place the light source in the light entrance socket of refractometer.
5. Look through the eyepiece and by a push/pull action; focus so that the light/dark demarcation
line can be seen in the field of view against the refractive index scale.
6. Read the refractive index of the gemstone from the scale where the demarcation line crosses
the refractive index scale.
Anisotropic Gemstones (Doubly Refractive)
Anisotropic gemstones will display two demarcation lines when viewed through the eyepiece.
These stones are sometimes known as ‘bi-fringent’ or ‘doubly refractive’. There are two types of
Anisotropic gemstones: (1) uniaxial, which has a single optical axis that does not cause light to
doubly refract, and (2) biaxial, which has two optical axes that do not cause the light to doubly
refract.
It is possible to identify whether a stone is uniaxial or biaxial by using the polariser and/or by rotating
the gemstone through steps of 90°, taking refractive index measurements and monitoring the
movement or prominence of the demarcation lines. Simple differentiation between uniaxial
gemstones and biaxial gemstones can easily be achieved by rotating either the polariser or
gemstone, where one demarcation line will remain static (uniaxial) as opposed to both demarcation
lines varying in position (biaxial). More complex determination can be achieved by measuring the
angle of double refraction as detailed below:
Using the polariser to identify Anisotropic Gemstones
1.
Place the gemstone on the refractometer following the steps above.
2. Place the polariser on the eyepiece.
3. Turn the polariser in the eyepiece until a single borderline is seen (light is vibrating in one
direction only, eliminating the extra-ordinary rays caused by birefringence).
4. Read the refractive index.
5. Gently rotate the gemstone on the prism by 90° taking care not to scratch the prism.
6. Turn the polariser by 90°
7. Measure the refractive index of the light vibrating at 90° to the measurement in point 4.
8. Repeat steps 5 to 7 until the stone has passed through 180°
Interpretation of the results will identify the value of double refraction by calculating the difference
between the measurement group with the maximum refractive index and the measurement group
with the lowest refractive index and comparing to known data.
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