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VI. THE CELL DENSITY ESTIMATE

As will be further dealt with in the following section the question of how many cells there are per unit area has been the subject of most specular microscopic studies. In principle the estimate of the number of cells present per unit area, i.e. the cell density, is obtained by counting the number of cells within an area of known dimensions, and is usually expressed as cells per square millimeter. Depending on the magnification and the technique used, the area of the endothelium seen at a time with most specular microscopes is only a fraction of a square millimeter. The eventual estimate of the cell density is therefore very sensitive to counting errors. As long as these errors are only statistical in nature a small counting area may be compensated for by a large number of photographs. If some of the errors are systematical errors the counting technique is said to be biased and can not be compensated for by a large number of counts. One of the commonly used methods, which has been used for blood cell counting, proceeds as follows: A rectangle of known dimensions is superimposed on the photomicrograph. All cells totally within the frame and those touching two neighbouring sides, say, the upper and left side, are included in the count while those touching the lower and right side are excluded. One