The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Lens
need not be laid on the presence of a few bubbles, as when not present in very large numbers they may be disregarded. For what purpose is the lens required ? For ordinary land-scape work, architectural subjects, interiors, or portraits ? For landscape work pure and simple there are few lenses to equal the achromatic single landscape lens, which gives brilliant negatives; and although distortion is present it may be dis-regarded, and it is practically unnoticeable in small views. Some of these lenses are now made to work aplanatic with an aperture ofand are therefore of nearly the same rapidity as the rapid rectilinear, which is, however, the lens most used by amateurs. Few amateurs will require a portrait lens, as they are not only difficult to use properly, but are exceedingly limited in action, and also expensive. The question of how much view to include on a plate is another important consideration which should not be lost sight of (see Angle, Width of) ; for ordinary work it should never exceed 500 to 550, and 450 is decidedly better. If a much greater angle be included, the resulting pictures have a* distorted appearance, because it is extremely unlikely that the eye will be placed at a focal length of the lens from the picture. The glass of which lenses are made should be colourless; this can be tested by laying the lens upon a sheet of white paper and looking down through it. Some of the cheaper lenses are made of glass which is not colourless: and any colour, especially brown or yellow, will make the lens slow. Optical glass is made somewhat in the following manner: - Crucibles of fire-clay of particular form are raised to a white heat in a furnace, and when the fuel ceases to give off smoke, they are charged with the materials, and the heat is continued for eight or ten hours. The crucible is now raised to a white heat for four hours, and the mixture stirred with a bar of baked clay. Six times from hour to hour the mixture is stirred. The heat is then reduced, that the bubbles may rise, and again at the end of two hours the heat is raised to make the glass fluid; again stirred, and the crucible and the openings of the furnace closed and left for eight days to cool. The crucible is taken out and broken, and the glass is removed and divided into pieces. The divided glass is examined and sorted, the finest being retained for astronomical purposes, the second quality for photographic lenses, and the third for ordinary magnifying glasses, the rest
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