Identifier: sciencerecordcom1874beac (find matches)
Title: The Science record; a compendium of scientific progress and discovery
Year: 1872 (1870s)
Authors: Beach, Alfred Ely, 1826-1896
Subjects: Technology Industrial arts
Publisher: New York, Munn
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Smithsonian Libraries

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s was set up on its edge between firm supports, inthe middle of a large, nearly dark room. A lamp was setat one end of the room, so as to shine upon the ba k sur-face of the glass at right angles ; behind the glass wasplaced a large lens of short focus, so that the light of thelamp passed through both the lens and the disc, and cameto a focus at about an equal distance on the other side.The eye being placed exactly at this focus, that portion ofthe glass disc backed by the lens appeared as a brilliantlyilluminated surface, on which the slightest defects weremagnified in a startling degree. The minutest specks,bubbles, and scratches appeared as huge deformities, andany vein of unequal density would appear as a wave onthe bright surface. This first test having been successful on both glasses,that by polarized light was applied. To apply the test, theglasses were taken into the open air, and laid on theground upon a piece of black cloth. Each glass was then MECHANICS AND ENGINEERING. 329
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33° SCIENCE RECORD. viewed in such a way that the light of the sky or cloudsshould be reflected from its under surface, and reach theeye after twice traversing its thickness. This light wasviewed through a Nicols prism held in the hand, andturned round and round, the glass also being turned roundso that the light should be examined in all directions. Theresult indicated that the flint glass was perfectly uniform,while in the crown there were very slight circles of strainfrom the centre to the circumference. This defect wouldnot interfere with the usefulness of the glass, and curious-ly enough, it disappeared during the grinding, so thatwhen the lenses were completed, no trace of unequal an-nealing could be seen. The application of these tests was completed early inJanuary, 1872, and the work of grinding was immediatelycommenced. Owing to the great size of the glasses, thefirst rough grinding was done by machinery, the grind-stone being a rapidly revolving iron wheel, over which astre

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