The growth of microbial populations can be measured in a number of ways. Some methods measure cell numbers, other methods measure the population's total mass, which is directly proportional to cell numbers. The ideal dilution water is neutral in effect. It maintains bacterial populations without stimulating cell growth and reproduction, damaging cells or reducing their ability to survive, grow, or reproduce. Its basic purpose is to stimulate the conditions of the natural environment which are favorable to cell stability. The media used support the growth of most heterotrophic bacteria; therefore, the routinely performed test is called a heterotrophic plate count, previously named by standard plate count. The streak plate method is a qualitative, indirect detection of microorganisms. However, the pour plate and spread plate methods are quantitative, direct measurements of the bacterial growth. Bacterial growth can be measured by direct microscopic counts. A known volume of medium is introduced into a calibrated etched glass slide called the Petroff-Hausser counting chamber.