Letter: Ardran/Crooks penetrameter cassette - necessity for re-calibration.
We have frequently been asked if the penetrameter cassette described by us (1968) requires frequent re-calibration. When this cassette was first designed it was our opinion that calcium tungstate intensifying screens did not alter in speed with time or use unless obviously worn, abraided, stained or dirty; we still hold this view.
Checking diagnostic x-ray beam quality.
Abstract A radiographic method is described for comparing the effective kilovoltage and inherent nitration of diagnostic X-ray machines. The apparatus consists of a modified Xray cassette covered by a sheet of lead perforated with two rows of holes; one row is associated with a strip of slow intensifying screen and provides a reference exposure, the other row has a strip of fast intensifying screen and a copper step wedge. The thickness of copper needed to give a film density which matches that of the reference exposure is called the copper reference number and provides an index of the beam quality. The device is calibrated against a constant potential generator for which the kV values are accurately known. For determining the effective kV of machine the beam is attenuated and hardened at source by means of a filter of 1·5 mm of Cu, corresponding approximately to the filtration (of the primary beam) provided by an abdomen plus Bucky grid, plus table top. The method is relatively independent of processing,...
The Inca Occupation of the South Coast of Peru
like the others, however; it forks into several nearly parallel ravines of which the largest are Huayuri, Rio Grande-Palpa, Ingenio (called Collao in ancient times), Nasca proper (the ancient Caxamarca), Taruga, and Poroma. Due to peculiarities of the local topography, the irrigated areas in Ica and the ravines of Nasca are located inland, at the foot of the mountains, rather than at the mouths of the rivers as is more usual in Peruvian coastal valleys. No sharp natural or cultural boundary separates the valley of Chincha from the valleys of Cafiete (ancient Huarco and Runahuana), Asia, and Mala, the next valleys going up the coast to the northwest. At the other end of the area we are considering, however, there is a sharp break; beyond Yauca the Andean foothills reach out to the sea, natural conditions are quite different, and there are important differences in the archaeological picture as well. The historical sources which mention the south coast are not particularly helpful, except in so far as they provide us with a few dates. To begin with, no chronicler gives an explicit account of the political organization of the south coast at the time it came into contact with the Incas. It can, perhaps, be inferred from Cabello Balboa's statements about common policy toward the Inca invaders that there was some sort of political connection between the valleys of Mala, Cafiete, and Chincha, but the connection need have been no more than a temporary military alliance. Cabello makes no mention at all of Pisco. The valleys of Ica, Nasca, and 125