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paragraph, which is entirely distinct from the rest of the discussion, he uses an expression which is not sufficiently approximate; e. g., if the expression be taken to mean the wave-length as stated above, and accordingly used to compute the number of waves in a given length in the line of vision, it differs from the truth by , precisely doubling the result found otherwise.

We assert, then, that the theory of 1887 is correct to terms of the order retained, which were sufficient; that Dr. Hicks's theory agrees with it precisely as to numerical amount and sign[1] of the effect, and that a third examination of the theory gives results differing from those of the two others only by negligible terms of the third order.

  1. Taking into account a note in 'Nature,' vol. lxv. p. 343 (1902).