10.12.1 For transparent liquids, record the hydrometer reading
as the point on the hydrometer scale at which the principal
surface of the liquid cuts the scale by placing the eye slightly
below the level of the liquid and slowly raising it until the
surface, first seen as a distorted ellipse, appears to become a
straight line cutting the hydrometer scale (see Fig. 1).
10.12.2 For opaque liquids record the hydrometer reading at
the point on the hydrometer scale to which the sample rises, by
observing with the eye slightly above the plane of the surface
of the liquid (see Fig. 2).