PARTICLE-SIZE analysis by microscopical methods provides large samples of measurements which then need numerical analysis to give estimates of average diameters, specific surface, etc. When, as is often the case, particle sizes are log-normally distributed1–4, tedious summation of powers of the sizes can be avoided if the cumulative distribution is plotted on log-probability paper1,4. From this, estimates of the mean, ȳ, and standard deviation, σ, of the distribution of log sizes can be read off; from these two values can be calculated any of the many kinds of average which are used in this work. If D be such an average, the general equation for this purpose is: where the value of c depends only on the kind of average, D, that is being calculated1,4.