This is an image of a quartz wedge viewed between crossed nicols.  The increase in thickness of the quartz (birefringence = 0.009) from upper left to lower right produces the range of interference colours from black (retardation of zero) in the upper left to fifth order red (retardation near 2750 mµ) in the lower right corner.  Above second order, the colours become alternating bands of pastel pink and green.  With increasing retardation, these eventually merge into one another, creating a colour known as “high order white”, at retardation values above the quartz wedge illustrated here.  This image may be useful in assessing the colours of the other images in this atlas as they appear on various computer monitors, by comparing the quartz wedge colours seen here with those produced by a quartz wedge in a lab setting.