Page explaining and illustrating w:astigmatism. Frederic Eugene Wright, The Methods of Petrographic-microscopic Research, Their Relative Accuracy and Range of Application, Carnegie institution of Washington, 1911 online
Page explaining and illustrating astigmatism. Frederic Eugene Wright, ''The Methods of Petrographic-microscopic Research, Their Relative Accuracy and Range of Application'', Carnegie institution of Washington, 1911.