THE characteristic curve of " Duplitized " X-ray film does not show a straight line por~ion in the range of useful densities, the average slope of the curve over the density range 0. 5-2.0 is taken as a measure of the contrast, and speed is proportional to the reciprocal of the exposure required to give a density of one. with standard development.
Con.trast is found to increase with development time to a maxinmm and then decreases owing to the growth of fog over the lower densities. The best development time is that giving the maximum contrast, and there is a range of development times on either side of this value for which the contrast does not differ greatly from the maximum. As a rule, the lower the fog. the greater is the contrast and the longer is the range of satisfactory development times. The relation between exposure and development time to give equal dens!ty of radiograph is shown graphically.
The correct time of development with a partially exhausted X-ray developer can be determined by measuring the time of appearance and multiplying by the appropriate ~Vatkin's factor.
X-ray films can be developed at temperatures up to 9o ยฐ F. by the use of hardening bath between development and fixing. Detailed directions for this process are given.