Questions 30-40

　　Most sources of illumination generate light over an appreciable period, and indeed if

　　an object is lit for a very brief time(less that 1/25 second), the human eye will not react

　　in time to see the object. A photographic emulsion---that is, a light-sensitive coating on photographic film, paper, or glass---will, however, record much shorter bursts of light. A

　　(5 ) photographic flash can therefore be used to capture high-speed movement on film as well

　　as to correct deficiencies of the normal surrounding lighting. Photoflash is now generated electronically, but the earliest form, first used in 1864, was a paper bag containing

　　magnesium wire and some oxygen-rich substance, such as potassium chlorate. When the

　　bag was ignited, the metal burned with an intense flash. A contemporary observer reported

　　(10) that “this quite unsafe device seems to have done nothing worse that engulf the room in

　　dense smoke and lead to pictures of dubious quality and odd poses.”

　　The evolution of the photoflash was slow, flashbulbs, containing fine wire made of a

　　metal, such as magnesium or aluminum, capable of being ignited in an atmosphere of pure oxygen at low pressure, were introduced only in the 1920’s. In the earliest type, the metal

　　(15) was separated from the oxygen by a thin glass bulb. The flash was fired by piercing the

　　bulb and allowing the oxygen to come into contact with the metal, which ignited

　　spontaneously. Later bulbs were fired by an electric battery, which heated the wire by

　　passing a small current through it. Other combinations, such as the pairing of oxygen

　　difluoride with zirconium, have also been used. In each case enough energy is given out to

　　(20) heat the oxidizable metal momentarily to a white-hot emission of visible light. The smoke particles are so small that they cool rapidly; but since they are white, they contribute to the brilliance by reflecting the light from their still-glowing neighbors. A slightly bigger form

　　of the metal will burn for a longer time.

　　30 What does the passage mainly discuss?

　　(A) The history of the photoflash

　　(B) Theories about how the eye reacts to light

　　(C) The technology of modern photography

　　(D) The dangers of using the early photoflash

　　31 According to the passage,1/25 second is the minimum amount of time required for the

　　(A) recording of an image on film

　　(B) generation of artificial light

　　(C) creation of a photographic emulsion

　　(D) human eye to react to light

(实习编辑：武勐勐)