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70 the abscissæ stand for the wave-lengths and the ordinates for the indices diminished by unity.

Each of the divisions marked on the axis of abscissae corresponds to 0.001, and each of the divisions marked on the ordinate axis corresponds to an excess over unity equal to 0.01.

In spite of all the care with which the experiments were executed, the deviations are so small, and, consequently, the indices so near to unity, that the table and diagram can only be regarded as a preliminary indication of the behaviour of the dispersion in the very slightly deviated part of the spectrum. An important consequence arises from these measures, viz. points corresponding to "N" rays, and those corresponding to N$1$ rays, are all situate on the