Limiting Index

It was shown by by Cauchy that the absolute refractive index of a medium might be expre ed by the formula, u=A+AS+A,S4, in which S is inversely proportional to the wave length in vacuum. As a consequence of this Prof. Powell pointed out that, as A diminishes, the wave length increases, the value of u approximates to a fixed limit given by the equation u2=A1; which therefore defines the limit of the spectrum at the red end. u, as given by the equation u2-A,, is called the limiting index. Langley has shown this conclusion to be incorrect.