682 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS sensitivity and threshold setting and counting those features which occur within a range of size, rather than measuring the exact size of each feature. The accuracy of counting with the Quantimet The accuracy with which the Quantimet is able to count particles has been tested using a specially designed test chart. Such a chart has been used previously to evaluate the Mullard Type L188 automatic particle analyser (12). The chart is shown in Fig. 11 the actual image examined in the microscope measured 2 mm2 and was produced photographically. o.e ß ß .0.1 Figure 11. The test chart. (Crown copyright reserved). The results are shown graphically in Fig. 18 where they are compared with the visual count, and the results obtained using the Mullard instru- ment (12). They are presented in the form of cumulative oversize as a per- centage of the total number, and are plotted using a Rosin-Rammler axis. It can be seen that the Quantimet gives an extremely representative result, and is more accurate than the Mullard instrument.

TttE PARTICLE SIZE ANALYSIS OF PIGMENTS WITH THE QUANTIMET ROSIN- RAMMLER AXIS MANUAL COUN.r 'QUAN'rlME'r' COUNT 'MULLARD' COUNT 100 1000 PARTICLE DIAME7ER Figure 12. A comparison of visual and automatic counters using the Rosin-Rammler distribution. Short and long term instrument stability Experience has shown that the Quantimet is very stable over periods of a few hours. This period can be considerably extended by maintaining adequate control procedures during operation. Of these, the most important sources of variation are camera sensitivity (which may be controlled automatically or manually, and can in any case be read out on the meter) and threshold setting. The possibility of such variation occurring can be reduced by standardization of technique. Nevertheless, variation is still experienced on a long term basis. The principal source of this occurs as a result of changing microscope objectives and magnification, with the necessitated changes in the microscope con- denser configuration (focus and aperture). These can be eliminated by ensuring that all samples which are to be directly compared with one another are measured before the instrumental conditions are changed. If this is impossible, then comparisons including separate results should be made via a common control which has been evaluated on both occasions. The Plastics Division instrument has now been in continuous use for 2} yr (actual usage 2 000 h) and has proved to be reliable. Failures were