Note 72 — Chapter III · Investigation of Microscopic and Optical Characters of Minerals.
Reading edition
The optical character may also be determined in parallel light by proving ć = c(+), ć = a(−). The optical character of the principal zone or the sign of the elongation is often given in tables. This optical character or sign is (+) when the principal zone axis or the direction of elongation is parallel to c and (−) when parallel to a.