National Geographic : 1943 Jan
The National Geographic Magazine of optical glass for use in 4,000 instruments. More spectacular to watch, from fiery fur nace to nosepiece, is the making of spectacle lenses, for ophthalmic glass is poured molten white from the pot onto a metal table where it cools off to red hot, then lightens to glassy semblance as it is rolled into sheets of required thickness. These sheets are measured, cut, inspected, and automatically weighed for the pressing room, where the pieces are reheated and given their first form. The grinding room is the dizziest motion picture of the glass industry. One grows sea sick watching row upon row of gyrating ma chines pitching and tossing what look like shells of a thousand turtles. The restive ob jects are lens blanks mounted in hot pitch on an iron block of prescribed curvature with a corresponding shell. Rouge Makes Many Passes at Girls Who Wear Glasses In the polishing room the annoying iron which was so painstakingly banished from the glass literally thumbs the nose of the lenses. For there a rouge, chemically like iron rust, polishes the ground lenses. A young woman touches up her cheeks with rouge from a compact; on her eyeglasses simi lar rouge has been applied with a tool impreg nated with wax which must be washed every time it is used. Bifocal lenses afford one example of why glass of special qualities is required for specific uses. Fused bifocals are made of two types of glass, having different refractive indices. Yet each glass must have the same coefficient of expansion at all stages of heating and reheating, and also when completed for wearing. One type of bifocals requires 62 different operations by 62 different persons and 26 different machines. Such refinements would have baffled even Benjamin Franklin, who is credited with first wearing bifocals. Some spectacle frames have from 20 to 25 units, and these require upwards of 500 opera tions for their manufacture and assembly. If your optician has to send away for lenses to fill an out-of-the-ordinary prescription, that is not surprising. He can scarcely carry 26,336 varieties of lenses in stock, as one big manufacturer does. Every person's eyes are different, as are thumbprints. So even the maker of lenses often is called upon for specials for extra ordinary cases. William Caxton, the Wright brothers, and the Industrial Revolution brought new de mands on the human eye by their contribu tions to printing, flying, and precision ma chinery. Constant eye research brings new problems for the makers of ophthalmic glass. Take aniseikonia, new word for a recently discovered eye defect. Aniseikonia is present when images from the two eyes, as interpreted by the brain, are unequal. Airplane pilots and motorists so afflicted are a menace to themselves and others. To detect and measure aniseikonia scien tists of the American Optical Company and Dartmouth Eye Institute developed an instru ment called the Eikonometer, which literally reads unequal images formed inside the brain. Then it was necessary to devise a new type of "tailor-made" lens to produce equal images in both eyes. Mention was made of the mathematics of optics. One investigator estimated that in a normal lifetime the average American reads 210 mil lion words, the equivalent of some 4,200 standard-length novels. 0.000,000,000,000,000,000,075 Horsepower Another calculated that the maximum amount of green light (which evokes greatest response in the retina) for reading fine print represents energy equivalent to 0.000,000, 000,000,000,000,075 horsepower. A third determined that the amount of energy required to excite the retina is equiv alent to that which would raise the tempera ture of one-fifteenth of an ounce of water to 1 degree, Fahrenheit, if continuously expended for 60 million years. Such tenuous calculations have no partic ular application; they are just the daydream doodling of men who grind lens surfaces for highly practical uses down to an accuracy of six millionths of an inch. Eight thousand words, and the epic story of glass gone to war is yet to be written! Wisely, the censor will not let it be told now. Glass is vital to World War II, as never be fore in the ageless history of wars. Withdraw glass and all airplanes would be grounded, radios silenced, lighthouses and beacons blinded, aerial mapping stopped. Trains would grind to a standstill, cameras would cease to click, ships would drift into port if they found their way there, big guns would be aimless. Numerous new applications of glass in war time will be adapted in startling ways to peace pursuits when the war is won.