The
results of this work have not as yet been published; but it is not
probable that the degree of accuracy attained is much greater than one
part in fifty or a hundred thousand. More recently, Mr. Bell, of the
Johns Hopkins University, using Rowland's gratings, has made a
determination of the length of the wave of sodium light which is claimed
to be accurate to one two hundred thousandth part[2]. If this claim is
justified, it is probably very near the limit of accuracy of which the
method admits. A short time before this, another method was proposed by
Mace de Lepinay.[3] This consists in the calculation of the number of
wave lengths between two surfaces of a cube of quartz. Besides the
spectroscopic observations of Talbot's fringes, the method involves the
measurement of the index of refraction and of the density of quartz, and
it is not surprising that the degree of accuracy attained was only one
in fifty thousand.
[Footnote 1: Nature, xx, 99, 1879; this Journal, III, xviii, 51, 1879.]
[Footnote 2: On the absolute wave lengths of light, this Journal,
III, xxxiii, 167, 1887.]
[Footnote 3: Comptes Rendus, cii, 1153, 1886; Journal, de Phys.,
II, v, 411, 1886.]
Several years ago, a method suggested itself which seemed likely to
furnish results much more accurate than either of the foregoing, and
some preliminary experiments made in June have confirmed the
anticipation.
Pages:
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135