revolving mirror started. If the image did not appear, the mirror was
inclined forward or backward till it came in sight.

[Footnote 3: The deflection being measured by its tangent, it was
necessary that the scale should be at right angles to the radius (the
radius drawn from the mirror to one or the other end of that part of
the scale which represents this tangent). This was done by setting the
eye-piece approximately to the expected deflection, and turning the
whole micrometer about a vertical axis till the cross-hair bisected the
circular field of light reflected from the revolving mirror. The axis
of the eye-piece being at right angles to the scale, the latter would
be at right angles to radius drawn to the cross-hair.]

The cord connected with the valve was pulled right or left till the images
of the revolving mirror, represented by the two bright round spots to the
left of the cross-hair, came to rest. Then the screw was turned till the
cross-hair bisected the deflected image of the slit. This was repeated
till ten observations were taken, when the mirror was stopped, temperature
noted, and beats counted. This was called a set of observations. Usually
five such sets were taken morning and evening.

[Illustration: FIG. 13.]

Fig. 13 represents the appearance of the image of the slit as seen in the
eye-piece magnified about five times.




Determination of The Constants.