U
]
There is a slight difficulty to be noted in consequence of the fact that
there are two series of waves in sodium light. The result of this
superposition of these is that as the difference of path increases, the
interference becomes less distinct and finally disappears, reappears,
and has a maximum of distinctness again, when the difference of path is
an exact multiple of both wave lengths. Thus there is an alternation of
distinct interference fringes with uniform illumination. If the length
to be measured, the centimeter for instance, is such that the
interference does not fall exactly at the maximum--to one side by, say,
one-tenth the distance between two maxima, there would be an error of
one-twentieth of a wave length requiring an arithmetical correction.
Among other substances tried in the preliminary experiments were
thallium, lithium, and hydrogen. All of these gave interference up to
fifty to one hundred thousand wave lengths, and could therefore all be
used as checks on the determination with sodium. It may be noted that in
case of the red hydrogen line, the interference phenomena disappeared at
about 15,000 wave lengths, and again at about 45,000 wave lengths; so
that the red hydrogen line must be a double line with the components
about one-sixtieth as distant as the sodium lines.
Pages:
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140