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Now, this sort of spectrum is greatly weakened by every increase of dispersive power, because the light is spread out into a longer ribbon and made to cover a more extended area.

On the other hand, a spectrum of bright lines undergoes no such weakening by an increase in the dispersive power of the spectroscope.

As it possesses high refractive and dispersive power, it presents when properly cut great brilliancy and “fire,” and the name has reference to its diamond-like appearance.

Homogeneity.—The optical desideratum is uniformity of refractive index and dispersive power throughout the mass of the glass.

On the other hand, the more extreme the optical properties of these new glasses, i.e. the further they depart from the ratio of refractive index to dispersive power found in the older glasses, the greater the difficulty found in obtaining them of either sufficient purity or stability to be of practical use.