Tests made on a clean concave speculum metal grating show that if the surface is coated with aluminum by evaporation, the time required to obtain a photograph of a spectrum is reduced by a factor which is the ratio of the reflection coefficient of aluminum to that of the speculum metal. The speed of the original grating was increased for the given wave-lengths by the following amounts:4077A–45 percent, 3341A–80 percent, and 2536A–170 percent. In preliminary tests the observed increase in speed was very much greater than indicated by these values, since the initial tests were made before the grating had been sufficiently cleaned.
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