The Electron Microscope The general layout of the illumination system and lenses of the electron microscope corresponds to the layout of the light microscope. The electrons are accelerated by a high-voltage potential (usually 40, 000 to 100, 000 volts), and pass through a condenser lens system usually composed of two magnetic lenses. The system concentrates the beam on to the specimen, and the objective lens provides the primary magnification. The final image in the electron microscope must be projected on to a phosphor-coated screen so that it can be seen. For this reason, the lenses that are the equivalent of the eyepiece in an optical microscope are called “projector“ lenses. Normally, the electron microscope is upside-down when compared with the light microscope, with the electron gun at the top of the column and the fluorescent screen at the bottom. The screen is viewed through a window let into the front. The column of the microscope is held under high vacuum to prevent the electrons passing through it from striking air molecules and being scattered. The strength of an electron lens depends on the current passing through the coil that produces the magnetic field. The strength of the lens can be varied by altering the current. In the electron microscope, therefore, the lenses are fixed, and adjustments are made to magnification and focus by altering the current passing through the lens coils. The condenser lens focuses the beam of electrons on to the specimen and affects the amount of illumination on the screen; the objective lens focuses the image; and the projector lenses alter the magnification. Magnetic lenses suffer from the same defects (chromatic and spherical aberration) in the same way as glass lenses. But the same methods of correction cannot be used, because there is no “negative“ electron lens. A very small lens aperture is employed to correct spherical aberration, but this severely limits the final resolution. Chromatic aberration is reduced by using electrons of a single wavelength. To produce such electrons, the accelerating voltage must be kept very steady because the wavelength of the beam is related to the accelerating voltage. Electron-microscope lenses suffer from the further aberration of astigmatism, which affects light-microscope lenses to a far lesser degree. Astigmatism is caused by the lens having two focal planes for axes at right angles to each other . Nothing can be done about astigmatism in an optical microscope. But in an electron microscope it can be corrected. Astigmatism in the electron microscope arises from two sources: from the lenses themselves, and from dirty apertures ( which are only 25 to 50 microns in diameter) in the objective lens. In both cases the astigmatism can be corrected by a skilled operator. In effect, the electron microscope achieves its superior resolution more in spite of, than because of, its lenses.

数字、模拟和混合计算机数字计算机是对离散数据进行操作的计算设备。它直接对表示数字、字母或其它专用符号的数进行计算。正如数字表以秒、分来计算小时那样，数字处理器也对离散数值进行计算以获得所需的输出结果。还存在一种与数字计算机相反的模拟计算机，它不直接对数进行计算。模拟计算机处理在连续标度上测量的变量并按预先规定的精度进行记录。例如：测量时，温度可精确到1／10摄氏度，电压可精确到1／100伏，压力可精确到“磅／平方英寸”。模拟计算机系统常用于控制诸如炼油厂中的流量和温度测量过程。有时把模拟和数字计算机的优点结合起来形成一个混合计算机系统。例如在医院重症监护室中，模拟部件可用于测量病号的心功能、体温和其它生理症状。然后把这些测量信号转换成数字并送给系统的数字部件，该部件用于监视病员生理症状，当检测到异常信号时，向护士值班室发出报警信号。很明显，模拟和混合处理器用于完成重要的特定任务。但绝大多数商业和科学用的计算机是数字计算机。

Our world has never been so small or so flat, thanks to technologies that have altered our relationship with time and space in ways that would startle even Albert Einstein.

More people travel between those cities by rail than by car and airplane combined.

We’ll create highly-skilled construction and operating jobs, and generate demand for technology that gives a new generation of innovators and entrepreneurs the opportunity to step up and lead the way in the 21st century.

Electric Traction It is generally held that the most efficient method of railway operation, and ultimately the most economical, given a reasonably cheap electricity supply, is with electricity as the motive power. The electric locomotive is not dependent, like its steam counterpart, on the competence of driving and firing or the quality of the fuel burned. On the other hand, the speeds that will be possible with any given load are completely predictable and apart from signal or permanent way checks, or other delay-producing casualties, exact observance of schedule times can be guaranteed within narrow limits. With such accurate timetable observance trains can be operated with shorter turnaround times at terminals, which means that more intensive use can be made of rolling stock than is generally possible with steam power. In suburban areas the rapid acceleration afforded by electric motors from rest and in recovery from speed restrictions makes it possible to work trains with very frequent stops at higher speeds and a closer headway than with steam locomotives. Multiple-unit working, which brings the motors of two or more sets of passenger stock, or two of more locomotives, under the control of one motorman in the leading driving cabin, would be impossible with steam, for each steam locomotive must be manned by its own driver and fireman. Moreover, except on the fastest and heaviest passenger and freight duties, on which it is advisable to have a second man at the head end to assist in the observation of signals and, if necessary, to attend to the electrical equipment, the equivalent of the steam locomotive fireman is unnecessary; thus the great majority of electric trains have a driver only, which makes for considerable economies in staffing. So much for the credit side of the ledger with electric operation. On the debit side is the extremely high cost of providing all the line equipment for supplying current to the trains, and of the sub-stations for feeding the current to the line conductors, hitherto this has been the chief obstacle to more widespread electrification. There is also the obvious disadvantage that, unlike the steam or diesel or diesel-electric locomotive, the electric motive-power unit cannot run anywhere beyond the line or lines equipped with conductors. Concentration of the power supply in large power-stations, also, means that any serious breakdown in the supply can have very widespread effects, bringing a large number of trains to a stand, though with the help of the modern grid system of current distribution in a country like Britain alternative sources of supply can generally be made available without much delay. There is the final disadvantage that in hard winter conditions icing of the conductors, which hinders the picking up of the current, may cause serious delays to trains, as has often been the case on winter mornings on the suburban lines round London; but such trouble is confined mainly to the railways on which the current is picked up from a third rail rather than from overhead conductors. These lines have to be provided with de-icing trains, which patrol them in the early mornings when icing conditions are severe, spreading certain liquid compounds on the rails in order to melt ice formations.

我们支持推动传统经济转型升级，挖掘绿色经济、蓝色经济、互联网经济等发展前景良好的新经济形态，走绿色、循环、低碳和高效发展之路。

稀土作为不可再生的战略资源，在新能源、新材料、节能环保、航空航天及电子信息等领域的应用日益广泛。

我们更加关注结构调整等长期问题，不随单项指标的短期小幅波动而起舞。

中国经济有巨大韧性、潜力和回旋余地。

我们采取的措施既利当前、更惠长远，有能力防范出现大的起伏，更不会发生“硬着陆”。

地铁施工在地铁施工中一直采用深挖法的原因是为了避免妨碍城市底下复杂的煤气、水管、电缆、电话线路及下水道系统。为了尽可能使地面上建筑物由于震动引起的损失减少到最小程度，地铁还要尽可能建造在主要街道中间部分的地底下，这就是为什么有些地铁系统有许多急弯的原因。有些地铁的大部分线路是靠完全临时控制有关街道建成的，从地面向下挖到铁路要求的深度，铺上轨道，然后又将所挖区域覆盖，恢复原来街道的面貌。这种方法叫做“明挖法”，当然这比挖地铁隧道对地面的干扰要大得多。与在粘土及类似土壤的机械化地铁挖掘截然不同的是在山区岩层挖掘隧道，这是所有挖掘工程中最花劳力和财力的。线路的每一英尺都要砸开。工人们使用风动和电动操作的凿岩机，在工作面上凿满了洞，将炸药装进去，夯实，随后他们必须撤到安全地方，直到通过电气插头把炸药引爆完毕。然后将砸开的岩石运走。接着整个过程重复进行，几乎是无休无止的，直到从岩石中开出一条线路来。岩石越硬，开凿工作就越困难，尽管如此，也很少有什么岩石坚硬到爆破后不用圬工或混凝土进行衬砌就可直接形成隧道壁。

What we need, then, is a smart transportation system equal to the needs of the 21st century.