This paper involves a problem confronted by the working diatom taxonomist who must make some basic observations and measurements of the diatom cell wall to achieve an accurate identification. the identification of diatoms to the species level is possible by microscopic observation and measurement of the intricate patterns on the cell wall. Since diatoms form the beginning of many aquatic food chains, they are important in many environmental studies and diatom communities can be used to assess the integrity or health of aquatic ecosystems. A morphological feature used in identification of centric diatoms is the number of striae found in a 10 micron arc along the edge of the diatom. This measurement is made by using a reticule in the microscope eyepiece. For large cells the arc and the 10 micron measuring cord become essentially identical. But for small cells the arc becomes increasingly larger than the 10 micron measuring cord, resulting in a distortion of the measured striae density. The solution of the problem of determining at what cell size a different method of determining the striae density must be used is presented.