The Preparation of Standard Glass Discs for Testing of Internal Strain in Glass

JSGT 1945 V29 T129-T147

Discs of window glass, 3½ inches in diameter, specially heat treated and then selected for uniformity of optical path difference, were mounted in sets of five and in that form were supplied by the Glass Container Association of America to all American glass-container manufacturers for use with the polariscope as comparison standards in grading glassware for annealing. The discs had a specified optical path difference of 22·76 µm ± 0·91 µm at points ¼ inch from the edge, as measured with monochromatic green light of wavelength 546·1 µm. The heat treatment was carried out in an electric furnace, and the selection for uniformity was based on measurements with a Goranson & Adams polarimeter reading to 0·30 µm. The theory and construction of the instrument are discussed. The range of variation of the optical path difference of the discs was estimated by measurement of the sources of error and by a statistical study of repeated measurements. The values lay in the range of ±1·52 µm or ±6·7% of the prescribed value of 22·76 µm.

T. D. Green