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Sheet of Glass
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cut into sheets by an electrical device and taken away to be cut into saleable sizes.
    SHEET GLASS PLANIMETRY. Sheet glass has a fire-polished surface; its thickness is determined by the skill of the operator or the accuracy of the machine, and as such the surface is infinitely superior to any artificially ground and polished surface, but it is definitely inferior to the artificial surface as regards planimetry. Perfect flatness would turn it into perfect plate glass and this is not yet within our scientific reach. The world demands an article with a higher degree of flatness than sheet glass for mirrors, motor-cars, shop windows, etc., and this is provided by ground and polished plate glass. For the purpose of measuring planimetry in comparative terms we use an optical method, depending on the distortion which can be observed by reflection from the surface under examination, of an illuminated screen ruled with squares.
    Figure 4a is reproduced from a photograph of the ruled screen reflected from the surface of a sheet of plate glass ¾" thick. Figure 4b is a similar photograph of the screen after reflection from a piece of quite good sheet glass, and by way of comparison Figure 4c is an ordinary ¼" plate glass. The amount of distortion might suggest that the departure from flat glass is considerable. The concavity shown here has in fact a radius of 300 feet.

P L A T E   G L A S S

The making of Plate Glass involves two processes:
(1) The manufacture of the rough glass blanks.
(2) The grinding and polishing of these rough blanks.