Page:Collected Physical Papers.djvu/86

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following investigations were made to study the effect of different strengths of solution, of temperature, and of different rays in the variation of the index.

It must be remembered that we are concerned here in measurement of effect of small variations from the standard. The absolute value of the index under the changed condition is deduced from the value under standard condition and the small observed variation.

2. Variation of the index with the different strengths of solution

The apparatus is first adjusted for total reflection with distilled water. To the distilled water in the cylinder is added enough salt (e.g., sodium chloride) to make, say, a five per cent. solution. This produces an increase of refractive power, and the angle of incidence has to be decreased to reach the critical angle. The difference of the two readings subtracted from the critical angle for distilled water (obtained from a previous accurate determination) gives the critical angle for five per cent. solution, and hence the absolute index. It is to be noticed here that a large number of determinations can be made rapidly by merely adding requisite quantities of salt and taking the corresponding readings.

With a finely-graduated circle, provided with a Vernier, the differences of readings may easily be obtained. With a roughly-graduated scale, the angular differences may be determined by fixing a mirror over the upper portion of the slit, and measuring the small differences in the usual way, by observing a reflected scale through a telescope, or receiving the reflected