before described. The heliostat was placed and adjusted. The revolving
mirror was inclined to the right or left, so that the _direct_ reflection
of light from the slit, which otherwise would flash into the eye-piece at
every revolution, fell either above or below the eye-piece.[2]

[Footnote 2: Otherwise this light would overpower that which forms the
image to be observed. As far as I am aware, Foucault does not speak of
this difficulty. If he allowed this light to interfere with the
brightness of the image, he neglected a most obvious advantage. If he
did incline the axis of the mirror to the right or left, he makes no
allowance for the error thus introduced.]

The revolving mirror was then adjusted by being moved about, and inclined
forward and backward, till the light was seen reflected back from the
distant mirror. This light was easily seen through the coat of silver on
the mirror.

The distance between the front face of the revolving mirror and the
cross-hair of the eye-piece was then measured by stretching from the one
to the other a steel tape, making the drop of the catenary about an inch,
as then the error caused by the stretch of the tape and that due to the
curve just counterbalance each other.

The position of the slit, if not determined before, was then found as
before described. The electric fork was started, the temperature noted,
and the sound-beats between it and the standard fork counted for 60
seconds. This was repeated two or three times before every set of
observations.

The eye-piece of the micrometer was then set approximately[3] and the