In order to get £ stationary with respect to the G point moving in x, y on the grating surface, all coefficients of the series must be set to zero whatever is the term power > 1, except for terms in x providing the dispersion by a linear displacement in x. Then, the variations of £ are d£ d£ n t— = constant, and t— = 0, dx dy which simultaneously entails the conditions

8 8' . , kk — + — = 0, and sin a + sin a = — . r r o

The latter relation is the grating equation, k is the diffraction order and a the line spacing. When substituting it in (3.75), the product Xx is constant. By denoting

1 cos a

a concise writing of the optical path length is

0 0

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