is used, and the image is observed through a direct vision spectroscope, interference bands are observed in the spectrum, which flit across it with the slightest rotation of the cylinder.
I give below the results obtained from determinations made on different days (temp. = 26°).
- (1) The larger apparatus was used for this experiment (diameter of the cylinder = 25 cm.), and the angle repeated fifteen times.
- (2) Experiment with the smaller apparatus (diameter of the cylinder = 14·4 cm.). Reading was only taken of two successive positions for total reflection with the air-film 'direct' and 'reverse.'
- (3) The same as above but with a different air-film.
- (1) Angle repeated fifteen times.
Mean value for
μ = 1·3321.
From (2) and (3), mean value of i obtained = 48° 40′.
The values of μ from the best determinations reduced to the temp. 26° are given below:—
Gladstone and Dale
Terquem and Trannin
The result obtained by me is thus seen to be practically the same as the above.
Having obtained the absolute value of the index of water for the D line and at the temperature of 26°, the