As the platinum-cobalt standard method is not well adapted for field
work, the color of the water to be tested may be compared with that of
glass disks held at the end of metallic tubes through which they are
viewed by looking toward a white surface. The glass disks are
individually calibrated to correspond with colors on the platinum scale.
Experience has shown that the glass disks used by the U. S. Geological
Survey give results in substantial agreement with those obtained by the
platinum determinations, and their use is recognized as a standard
procedure.