When the sum of the radii and the distance between the centres are equal, the circles touch externally. If the difference between the radii and the distance between the centres are equal, then the circles touch internally.
Zone system : is the method of determining exposure and development required for individual scenes, invented by Ansel Adams. It is based on analysis of subject luminosities in terms of ten gray tones, labeled zones 0 through X and previsualizing them as print densities. By measuring each subject luminance with a hand meter it is possible to determine how much the range of values must be contracted or expanded by negative development control to give the required values in the print.
Deoxyribonucleic Acid [DNA]
Deoxyribonucleic acid is a molecule that encodes the genetic instructions used in the development and functioning of all known living organisms and many viruses. DNA and RNA are nucleic acids; alongside proteins, they compose the three major macromolecules essential for all known forms of life.