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1907 Nat. Electr. Light Assoc. 30th Convention I. 337 The most sensitive flicker frequency for small ranges of flicker was in the neighborhood of 2.5 cycles per second.

1922 Jrnl. Optical Soc. Amer. 6 7 The substantial independence of critical flicker frequency upon chroma is..the basis of the critical frequency method of heterochromatic photometry.