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To the Editor:
—Apropos of Dr. Wilson's instructive article on the cross cylinder and the astigmatic chart (Wilson, A.: Combination of Cross Cylinder and Astigmatic Chart in Correction of Astigmatism, Arch. Ophth. 28: 490 [Sept.] 1942), the following considerations may be found helpful. There are a few points in the technic given which are not very clear, but the idea that one may use at option either plus or minus cylinders to equalize the clock lines is not always workable. In fact, whether to use plus cylinders or minus cylinders with the clock dial will depend on two factors : (1) whether the astigmatism is hyperopic or myopic, naturally or rendered so by auxiliary spheres, and (2) how the examiner wants to equalize the lines, that is, to equalize by clearing up the blurred lines or to equalize by blurring the clear lines. A fundamental principle in line chart tests is