This is the X-ray crystallography pattern of DNA, obtained by Rosalind Franklin and Raymond Gosling in 1952. It is known as the B-form. Both James Watson and Francis Crick were struck by the simplicity and symmetry of this pattern. The distinctive X in this X-ray photo is the tell-tale pattern of the helix. Because the X-ray pattern is so regular, the dimensions of the helix must also be consistent. For example, the diameter of the helix stays the same. Using this pattern, Rosalind Franklin and Morris Wilkins were able to derive some basic dimensions for the DNA structure. In an X-ray diffraction pattern, the closer the spots, the larger the actual distance. The horizontal bars actually correspond to helical turns. The vertical distance between the bars, 34 angstroms, is the measure of the height of one turn. The distance from the middle of the X-ray pattern to the top is measurable at 3.4 angstroms. This corresponds to the distance between two stacked base pairs. Since the height of one helical repeat is 34 angstroms and the distance between stacked base pairs is 3.4 angstroms, there must be 10 nucleotides per helical repeat. The pitch of helix, or the degree of rise, can be calculated from the angle, the x makes with the horizontal axis. By distorting the helix, you can get an idea how the helical pitch is related to the X-ray pattern. From this X-ray diffraction pattern, Franklin deduced that the phosphate groups were on the outside. From the measurement made by Rosalind Franklin the basic dimensions of the DNA structure were known.