Gypsum Plate

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“Determination of the optical character of a biaxial crystal in a section cut perpendicular to the acute bisectrix by means of the gypsum plate (+).” -Johannsen, 1908


Albert Johannsen A Key for the Determination of Rock-Forming Minerals in Thin Sections (New York, NY: John Wiley & Sons, 1908)


TIFF (full resolution)

2400×2241, 359.0 KiB

Large GIF

1024×956, 60.7 KiB

Medium GIF

640×597, 33.9 KiB

Small GIF

320×298, 13.5 KiB