Page:NBS Circular 553.djvu/20

 6.3.5. Fluorescent Solids or Liquids. The color of the fluorescence of a solid or liquid is determined by holding it or the test tube or vial directly in front of a  vertical black velvet cloth so that the sample is illuminated by natural or artificial daylight chiefly in directions perpendicular to the line of sight. Any light incident upon the sample from the back should be avoided because it will produce a mixture of fluorescence and body colors in unknown proportions. The fluorescence color will appear the strongest on the side toward the light source. The Munsell charts should receive light from the same source and should be held slightly tilted beside the sample to facilitate the comparison. The hue notation is sufficient.

6.4.1. Mounting. Unless otherwise specified, the sample whether powdered or sectional is to be mounted in a colorless medium such as water.

6.4.2. Lighting. A frosted Mazda lamp is used for illumination and is placed above and to one side of the microscope base where it will illuminate both the substage mirror of the microscope and the Munsell charts (figs. 9 and 10). The proper daylight filter (blue glass) for the light-source used must be placed in the eyepiece of the comparison ocular or the Abbé-type camera lucida, whichever is used.

6.4.3. Determination of Munsell Notation. When using a comparison ocular (fig. 9), place one end over the ocular of the microscope and support the other end by a condenser clamp. If an Abbe-type camera lucida is used (fig. 10), it must be modified somewhat to give a split field. To do this, cut out of a circular piece of tin foil just large enough to slip into the ocular tube of the microscope, a semicircle as shown in the accompanying diagram (fig. 11). This shield is placed on top of the ocular micrometer with the cut edge extending from back to front and constituting the dividing line. Next, place a piece of black velvet or cloth on the table beneath the reflecting mirror of the camera lucida so that its outer edge coincides with the vertical edge of the shield in the ocular tube. This cloth must cover everything on the table visible in the upper half of the reflecting mirror. Slip an envelope of black paper or cloth down over the top half of the reflecting mirror until its lower edge also coincides with the dividing line.

Now place a mount consisting of a slide, a sample (for instance a thin section), a coverglass and liquid Rh