_Procedure._—The color of a sample shall be observed by filling a standard Nessler tube to the height equal to that in the standard tubes with the sample and by comparing it with the standards. The observation shall be made by looking vertically downward through the tubes upon a white or mirrored surface placed at such angle that light is reflected upward through the column of liquid. Water that has a color greater than 70 shall be diluted before making the comparison, in order that no difficulties may be encountered in matching the hues. Water containing matter in suspension shall be filtered, before the color observation is made, until no visible turbidity remains. If the suspended matter is coarse, filter paper may be used for this purpose; if the suspended matter is fine, the use of a Berkefeld filter is recommended. The Pasteur filter shall not be used as it exerts a marked decolorizing action. The apparent color, if determined, shall be determined on the original sample without filtration. The true and the apparent color of clear waters or waters with low turbidities are substantially the same. The results of color determinations shall be expressed in whole numbers and recorded as follows: Color between 1 and 50 recorded to nearest unit " " 51 " 100 " " " 5 " " 101 " 250 " " " 10 " " 251 " 500 " " " 20. COMPARISON WITH GLASS DISKS.[105] 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. COMPARISON WITH NESSLER STANDARDS. Inasmuch as the Nessler scale[62] and the natural water scale[22][49] which agrees with it except for colors less than 20, have been largely used in the past, the old results may be converted[117] into terms of the platinum standard by means of the ratios in Table 3, but they must not be considered as universally applicable as the variable sensitiveness of the Nessler solution introduces an uncertain factor. Table 3.—VALUES FOR CONVERTING COLORS BY THE NATURAL WATER SCALE INTO COLORS BY THE PLATINUM STANDARD IN PARTS PER MILLION.[B]