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The word laser was coined as an acronym for Light Amplification by the Stimulated Emission of Radiation. Ordinary light, from the Sun or a light bulb, is emitted spontaneously, when atoms or molecules get rid of excess energy by themselves, without any outside intervention . Stimulated emission is different because it occurs when an atom or molecule holding onto excess energy has been stimulated to emit it as light. Albert Einstein was the first to suggest the existence of stimulated emission in a paper published in 1917. However , for many years physicists thought that atoms and molecules always were much more likely to emit light spontaneously and that stimulated emission thus always would be much weaker. It was not until after the Second World War that physicists began trying to make stimulated emission dominate. They sought ways by which one atom or molecule could stimulate many other to emit light , amplifying it to much higher powers. The first to succeed was Charles H.Townes, then at Colombia University in New York . Instead of working with light , however, he worked with microwaves, which have a much longer wavelength, and built a device he called a "maser" for Microwave Amplification by the Stimulated Emission of Radiation. Although he thought of the key idea in 1951, the first maser was not completed until a couple of years later. Before long, many other physicists were building masers and trying to discover how to produce stimulated emission at even shorter wavelength. The key concepts emerged about 1957. Townes and
Arthur Schawlow, then at Bell Telephone Laboratories, wrote
a long paper outlining the conditions needed to amplify
stimulated emission of visible light waves. At about the same
time, similar ideas crystallized in the mind of Gordon
Gould, then a 37- year-old graduate student at Columbia,
who wrote them down in a series of notebooks. Townes and Schawlow
published their ideas in a scientific journal, Physical Review
Letter, but Gould filed a patent application. Three decades
later, people still argue about who deserves the credit
for the concept of the laser. Panel painting, common in thirteenth -and fourteenth -century Europe , involved a painstaking , laborious process. Wooden planks were joined, covered with gesso to prepare the surface for painting , and then polished smooth with special tools. On this perfect surface, the artist would sketch a composition with chalk, refine it with inks, and then begin the deliberate process of applying thin layers of egg tempera paint (egg yolk in which pigments are suspended) with small brushes. The successive layering of these meticulously applied paints produced the final, translucent colors. Backgrounds of gold were made by carefully applying sheets of gold leaf, and then embellishing of decorating the gold leaf by punching it with a metal rod on which a pattern had been embossed. Every step in the process was slow and deliberate . The quick-drying tempera demanded that the artist know exactly where each stroke be placed before the brush met the panel, and it required the use of fine brushes. It was, therefore , an ideal technique for emphasizing the hard linear edges and pure, fine areas of color that were so much a part of the overall aesthetic of the time. The notion that an artist could or would dash off an idea in a fit of spontaneous inspiration was completely alien to these deliberately produced works. Furthermore, making these paintings was so time-consuming that it demanded assistance. All such work was done by collective enterprise in the workshops. The painter or master who is credited with having created painting may have designed the work and overseen its production, but it is highly unlikely that the artist's hand applied every stroke of the brush. More likely, numerous assistants, who had been trained to imitate the artist's style, applied the paint. The carpenter's shop probably provided the frame and perhaps supplied the panel, and yet another shop supplied the gold. Thus, not only many hands , but also many shops were involved in the final product. panel : n. 面板,嵌板,仪表板,座谈小组,全体陪审员;vt. 嵌镶板 Crows are probably the most frequently met and easily identifiable members of the native fauna of the United States . The great number of tales, legends, and myths about these birds indicates that people have been exceptionally interested in them for a long time. On the other hand, when it comes to substantive -- particularly behavioral -- information, crows are less well known than many comparably common species and, for that matter, not a few quite uncommon ones: the endangered California condor, to cite one obvious example. There are practical reasons for this. Crows are notoriously poor and aggravating subjects for field research. Keen observers and quick learners, they are astute about the intentions of other creatures, including researchers, and adept at avoiding them. Because they are so numerous, active, and monochromatic, it is difficult to distinguish one crow from another. Bands, radio transmitters, or other identifying devices can be attached to them , but this of course requires catching live crows, who are among the wariest and most untrappable of birds. Technical difficulties aside , crow research is daunting
because the ways of these birds are so complex and various.
As preeminent is generalists, members of this
species ingeniously exploit a great range of habitats
and resources, and they can quickly adjust to
changes in their circumstances. Being so educable,
individual birds have markedly different interests and
inclinations, strategies and scams. For
example, one pet crow learned how to let a dog out of its kennel
by pulling the pin on the door. When the dog escaped,
the bird went into the kennel and ate its food. In the early days of the United States, postal charges were paid by the recipient and Charges varied with the distance carried. In 1825, the United States Congress permitted local postmasters to give letters to mail carriers for home delivery, but these carriers received no government salary and their entire compensation depended on what they were paid by the recipients of individual letters. In 1847 the United States Post Office Department adopted the idea of a postage stamp, which of course simplified the payment for postal service but caused grumbling by those who did not like to prepay. Besides, the stamp covered only delivery to the post office and did not include carrying it to a private address. In Philadelphia, for example, with a population of 150,000, people still had to go to the post office to get their mail. The confusion and congestion of individual citizens looking for their letters was itself enough to discourage use of the mail. It is no wonder that, during the years of these cumbersome arrangements, private letter-carrying and express businesses developed. Although their activities were only semilegal, they thrived, and actually advertised that between Boston and Philadelphia they were a half-day speedier than the government mail. The government postal service lost volume to private competition and was not able to handle efficiently even the business it had. Finally, in 1863, Congress provided that the mail carriers
who delivered the mail from the post offices to private addresses
should receive a government salary, and that there should be
no extra charge for that delivery. But this delivery service
was at first confined to cities, and free home delivery
became a mark of urbanism. As late as 1887, a town had
to have 10,000 people to be eligible for free home delivery.
In 1890, of the 75 million people in the United States, fewer
than 20 million had mail delivered free to their doors. The
rest, nearly three-quarters of the population, still received
no mail unless they went to their post office. Archaeology has long been an accepted tool for
studying prehistoric cultures. Relatively recently
the same techniques have been systematically applied
to studies of the more immediate past. This has been called
"historical archaeology," a term that is used in the
United States to refer to any archaeological investigation
into North American sites that postdate the arrival of
Europeans. In Kingston, New York, for example, evidence has uncovered
that indicates that English goods were being smuggled
into that city at a time when the Dutch supposedly controlled
trading in the area. And in Sacramento an excavation
at site of a fashionable nineteenth-century hotel revealed
that garbage had been stashed in the building's
basement despite sanitation laws to the contrary.
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