The same mount is used in estimating the bacteria, but the ×18 ocular used so as to increase the magnification to approximately 500 diameters. The “number in several areas, each consisting of five of the small squares, is counted.” Nothing is said as to the order of the five squares, whether in a row or other arrangement, nor what number constitutes “several.” The average number found in five squares represents the number in 1-800,000 part of a cc, and this multiplied by 3, for the dilution, would make the factor 1-2,400,000 for a cc. It is stated that it is believed that it is possible for manufacturers to keep within 12,500,000 bacteria per cc in the pulp and 25,000,000 in ketchup. The number present is expressed in terms per cc though the yeast and spores are expressed in 1-60 c mm. Possibly bacteria to the lay mind mean something dangerous, so by expressing the numbers in millions they appear appalling. Yeasts and spores are not so generally associated with dirt and disease so that by giving them a small unit, only 1-60,000 part of a cc, they may seem much less offensive. If the mind is capable of conceiving what is meant by millions per cc for bacteria in one case, there seems to be no good reason why the same unit of volume should not hold for the other. To estimate the number of molds present, a drop of the undiluted pulp or ketchup is placed on an ordinary slide and the ordinary cover-glass pressed down until a film of 0.1 mm is obtained. The directions state that after some experience this can be done, but do not state how one’s efforts may be directed to obtain this result. It is apparent that by experience in comparing a measured amount with a judged amount that the tendency would be toward accuracy, but in this case there is no measured amount for comparison, except the diluted drop in the counting chamber. Some workers have placed thin cover-glasses under the edges of the mount so as to have something to help in estimating the thickness of the film, but as the thinnest ordinary cover-glasses vary from .12 to .17 mm in thickness, the error varies 20 to 70 per cent from that required. One manufacturer in advertising No. 1 cover-glasses states that they vary from 0.13 to 0.17 mm, while another states they vary from 1-200 to 1-150 of an inch (0.127 to 0.169 mm). Careful checks show that it is not always easy to get exactly .1 mm on the specially prepared counting chamber; that unless the cover be placed with care and pressed uniformly on all sides until Newton’s rings appear, a variation of ten per cent or more in thickness may occur, and without such a guide the error becomes greater. The micrometer screw adjustment on the microscope can be used to help in determining the thickness, but none of the workers observed has used this refinement.