It is found that the absolute refractive index of any optical glass studied attains a minimum value at some temperature, and that the minimum refractive index is attained at decidedly higher temperatures with long wave-lengths than with short wave-lengths. Absorption bands in the ultraviolet are known to advance toward longer wave-lengths when an absorbing glass is warmed from liquid air temperature. Because of its closer proximity, the absorption band’s advance would be accompanied by a more rapid increase of absolute refractive index N in the short wave end than in the long wave end of the spectrum, with rising temperature. At the same time expansion decreases the physical density and so tends to diminish N with rising temperature. At some particular temperature the two effects of temperature change may cancel each other. At lower temperatures than this, the density effect may prevail and N fall with rising temperature, while at higher temperatures than that for minimum N, the absorption effect may exceed the density effect, and the N must rise with rising temperature.
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