A new refractometer, Type Dokf-EK. 85, was made in 1985 by the united efforts of Doshisha University, Kyoto Fission-Track Ltd. and K. K. Eikodenki (see. Fig. 1). It can measure the refractive indices of individual glass flakes at intervals of 1×10^-4 under a microscope.
The index value of refraction follows the temperature change of immersion liquid, which is automatically controlled by the thermo module in this instrument. When the glass flakes in the immersion liquid disappear under the microscope, the operator sends a signal to the microcomputer. After the steps of procedure are repeated about 10 times on a glass flake, the microcomputer calculates its refractive index value as the mean of all measurements.
The refractive indices of volcanic glass flakes in the widespread tephras which are famous as the makers of datum plane in the Japanese Quaternary such as Akahoya, Aira-Tn, Pink and Azuki Volcanic Ash Layers, were measured. The obtained values from each tepha were well concordant with those in the previous reports (Table. 3).
This refractometer is useful for the identification of tephras, because it is very easy to measure the refractive indices of many glass flakes, and to discriminate none-essential glass under the microscope, even if they are of a very small amount and are mixed in other kinds of grains such as minerals and pumices.