The standard by which the density of smoke is to be measured is to the Ringelmann Chart, published by the United States Bureau of Mines. Use of the chart shall be made by placing it at such distance from the observer so that the squares appear as even shades of coloring or where no white space between the lines are visible; such observation may be made by the chart itself or by the use of an optical device approved by the Board of Health utilizing Ringelmann chart standards, and by this Part the optical device, being known as "Smokescope," manufactured by the Mine Safety Appliance Company of Pittsburgh, Pennsylvania, is hereby declared to be approved as an optical device for such testing. Comparison of the smoke under observation with the various shades of the chart shall then indicate the density of the smoke. Observation distances shall be not less than 100 feet nor more than 1/4 mile from the smoke observed.