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1. A photo-electric cell was used to measure photometrically the opacity of cultures of Salmonella gallinarum growing in bouillon.
2. The opacity of a culture was defined as the number of 24-hour-old bacteria which when suspended in the same type of bouillon as those under investigation would absorb the same amount of light. Seven experiments were made to compare the opacities of growing cultures with their actual numbers throughout the first 24 hours of growth. An additional experiment was made to find directly the numbers of bacteria of different ages which would absorb the same amount of light.
3. The ratio between opacity and number was greatest at about 3 hours, decreasing to unity at about 7 hours; the optical behaviour of cultures 7 hours old was thus the same as of cultures 24 hours old. It is possible from the results that the ratio decreased to a minimum at about 9 hours, thereafter again rising to unity. The value of the greatest ratio was greater as the inoculum was less. The mean greatest ratio was 4·6, the mean time being 3 hours. This means that it would take about 4·6 times as many 24-hour-old bacteria as 3-hour bacteria to absorb the same amount of light.
4. It was shown that the variation in the opacity-number ratio was in all likelihood due to changes in the size of the organism during growth.
5. The results suggest that the opacity curves give a truer picture of actual growth than the ordinary growth curves, since the cells change in size during growth.
