Note 116 — Chapter IV · The Microscopic and Optical Characters of Minerals. (continued 2)
Reading edition
In quartz ω is the refractive index of the ray with vibration direction ∥ a [that is the direction of vibration of the faster ray (the ordinary ray)]. Hence ω is direction ∥ a and ε ∥ c. In the feldspars; α ∥ a, γ ∥ c and β ∥ b.