CROSSLINE DISC, Whipple

Glass disc, with linear ruling or grid indelibly inscribed Fit over diaphragm in eyepiece NOTE: unless using precalibrated types under specified conditions, must calibrate discs by means of a stage micrometer to establish value of graduation interval for each combination of eyepiece, tube length, and objective magnification. Whipple Eyepiece Crossline Disc Design approved by Melville C. Whipple, Harvard University, Engineering School, Department of Sanitary Chemistry Diameter, 21 mm Used in water supply testing, in blood test for lead poisoning and in numerous techniques for counting bacteria, plankton, dust, etc.; see American Public Health Association, Standard Methods for the Examination of Water and Wastewater, Method 1002E. Large square divided into 100 small squares, one of which is further subdivided into 25 smaller squares with vertical and horizontal lines extending full width of large square. Can be used in Leica and Reichert-Jung Huygenian and Leica wide-field, eyepieces; does not fit Reichert-Jung wide-field eyepieces.