when slowly decreasing. Further, this error would form part of the
probable error deduced from the results of observations.

We may then conclude that the error, in the measurement of _n_, was less
than .00002.



The Deflection.


The angle of deflection φ was measured by its tangent, tan φ = d/r; d was
measured by the steel screw and brass scale, and r by the steel tape.

The value of one turn of the screw was found by comparison with the
standard meter for all parts of the screw. This measurement, including the
possible error of the copy of the standard meter, I estimate to be correct
to .00005 part. The instrument is at the Stevens Institute, where it is to
be compared with a millimeter scale made by Professor Rogers, of
Cambridge.

The deflection was read to within three or four hundredths of a turn at
each observation, and this error appears in the probable error of the
result.

The deflection is also affected by the inclination of the plane of
rotation to the horizon. This inclination was small, and its secant varies
slowly, so that any slight error in this angle would not appreciably
affect the result.