Positive crystals

(Opt.) a doubly refracting crystal in which the index of refraction for the extraordinary ray is greater than for the ordinary ray, and the former is refracted nearer to the axis than the latter, as quartz and ice; - opposed to negative crystal, or one in which this characteristic is reversed, as Iceland spar, tourmaline, etc.

See also: Positive

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
References in periodicals archive ?
Needle-like positive crystals (type III) are observed if crystallization occurs at 300 [degrees] C.