sides of this central diaphragm are cemented, as previously stated, the two glass slips which contain the air-film.
One set of readings having been obtained with the semi-cylinder P turned towards the source of light, the cylinder is turned round through 180°, and observation repeated with Q occupying the previous position of P; it will be seen that by this procedure the air-film is also reversed. The mean value of the critical angle obtained from the two sets of observations, one "direct," the other "reverse," is thus free from any outstanding error.
Having explained the principle of measurement, I now proceed to give an account of the experiments, and compare the result obtained by this method with those obtained by previous observers. The values for carbon disulphide, absolute alcohol, glycerine have been determined, but the results obtained by different observers do not agree. The reason for this discrepancy is obvious; it is almost impossible to obtain two different specimens of these substances exactly alike. The only substance which can be obtained in a state of approximate purity is distilled water, but even here we have various contaminations by the absorption of different gases like ammonia and carbonic acid from the atmosphere; or glass itself may be dissolved in minute qualities. The values of the index for water obtained by different observers are therefore not very concordant. The following are the values of the index of water for the D ray:—
Wollaston and Brewster, D ray (temp. not given)
Sir John Herschel (at density of 1)