41. The photograms or photographic plates are oriented by measured passpoints in the field or on the building. 42. The collodion process was used on a glass photographic plate to capture an image on the plate. 43. In the early days of the 20th century he used photographic plates , like all other serious photographers. 44. The seated man is taking a radiograph of his hand by placing it on a photographic plate . 45. The manufacture of photographic plates has been discontinued by Kodak, Agfa and other widely known traditional makers. 46. The world's astronomical archives contain more than 2 million photographic plates , some more than a century old. 47. His early work was focused on astrometrics, particularly in the examination of photographic plates to measure stellar parallax. 48. In that time, he had a small laboratory where he could develop photographic plates or build radio equipment. 49. He spearheaded a revolution in which astronomers went from using photographic plates to much more efficient electronic detectors. 50. But until the 1990s, most observations were recorded on photographic plates and so are not available in digital form.
