It was found possible
to increase this number to more than 200,000. Now it is very easy to
estimate tenths or even twentieths of a wave length, which implies that
it is possible to find the number of wave lengths in a given fixed
distance between two planes with an error less than one part in two
millions and probably one in ten millions. But the distance
corresponding to 400,000 wave lengths is roughly a decimeter, and this
cannot be determined or reproduced more accurately than say to one part
in 500,000. So it would be necessary to increase this distance. This
can be done by using the same instrument together with a comparer.
The intermediate standard decimeter, lm (Fig. 2), is put in place of
the mirror, b. It consists of a prism of glass one decimeter long with
one end, l, plane, and the other slightly convex, so that when it
touches the plane, m, Newton's rings appear, and these serve to
control any change in the distance, lm, which has been previously
determined in wave lengths.
The end, l, is now adjusted so that colored fringes appear in white
light. These can be measured to within one-twentieth of a wave length,
and probably to within one-fiftieth. The piece, lm, is then moved
forward till the fringes again appear at m. Then the refractometer is
moved in the same direction till the fringes appear again at l, and so
on till the whole meter has been stepped off.
Pages:
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137