学生版lecture2

1、The RetinaWithin the choroid covering the posterior two-thirds of the eyeball is the final destination of the light, the light-sensitive layer called the retina. The purpose of everything in front of the retina is ultimately to form an appropriate real image of the outside world on this layer. This

2、image, like that of a single thin lens, is inverted on the retina. The effective focal length of the relaxed eye is about 17mm, which means that the retinal image is the same size as that which would be formed by a single 17mm focal length lens. When the eye is fully accommodated for nearby objects,

3、 the effective focal length shrinks to about 14mm.Often when people first learn of the inverted retinal image, they are puzzled as to why we do not see the world upside down. However, a little contemplation will clear up the mystery.Seeing is emphatically not just the passive transfer of the retinal

4、 image to some portion of the brain for inspection by some kind of little demon there (after all, how would the demon “see” it?). The retinal image has always been inverted; usually we have had no experience of anything else. The conscious “picture” we see is simply the way the brain interprets the

5、retinal image as it exists. Along these lines, some very interesting experiments have been done with human subjects wearing inverting goggles. The American psychologist G. M. Stratton actually wore an image-inverting device for 8 days as long ago as the 1890s. As far as we know he was the first pers

6、on to see with a noninverted, or right-side-up, the retinal image. Of course, ordinary actions were difficult at first, but by the fifth day things seemed almost normal although his views of parts of his own body still seemed confused. When bewildering but not upside down; the strangeness lasted onl

7、y a few hours. Apparently, the visual system can learn to function with very different kinds of retinal images, even those distorted in some complex ways. Nearsightedness and FarsightednessTwo common visual defects result from failure of the eye to focus images precisely at the position of the retin

8、a. Nearsightedness, or myopia, occurs when the eyeball is longer than normal so that the relaxed eye forms the image of a distant object in front of the retina. Farsightedness, or hyperopia, occurs when the eyeball is shorter than normal so that the relaxed eye forms the image of a distant object be

9、hind the retina. The figure below illustrates these conditions. Sometimes the word hypermetropia is used in place of hyperopia. Actually the nearsighted person can see nearby things fairly clear; we could say the myoic persons far point (the farthest away one can focus) is less than infinity. You ca

10、n understand this result if you remember our earlier discussion of the image-forming properties of lenses; as an object is moved closer to a lens, its real image moves farther away. An object close enough to the relaxed myopic eye will produce an image on the retina, and any object closer can then b

11、e focused by accommodation. The opposite is true for the hyperopic person; distant objects can, be seen clearly, but the near point is abnormally far away. In this respect the figure above is somewhat misleading because the eye is shown relaxed, whereas the hyperopic person would actually accommodat

12、e slightly(fatten the lens) to bring the image onto the retina. Only in this case, because of the shortness of the eyeball, full accommodation will bring into focus only things relatively far away; any closer object will be blurred. Note that both the presbyopic and hyperopic eye are characterized b

13、y abnormally distant near points and could be called farsighted, but for different reasons, the former because of the lens, the latter because of the shape of the eyeball. The nearsighted eye can be corrected by diverging lens and the farsighted eye by a converging lens. Exercise I. Read the followi

14、ng statements carefully and decide whether they are true or false according to the text.1. The image is inverted on the retina. 2. When the eye is completely adjusted for distant objects, the effective focal length reduces to approximately 14mm.3. The retinal image has always been upside-down and pe

15、ople often have had no experience of anything else.4. The American psychologist G. M. Stratton actually wore a inverting goggles for 192 hours as long ago as 1890s.5. Myopia and hyperopia are caused because the eye cannot focus images precisely at the position of retina.6. Farsightedness occurs when

16、 the eyeball is longer than normal so that the relaxed eye forms the image of a distant object behind the retina.7. Hypermetropia is also called hyperopia.8. Nearsighted peoples far point is less than infinity. 9 Farsighted person can see distant things fairly clearly, but the near point is abnormal

17、ly far away.10. The preshyopic is characterized by abnormally distant far points and could be called farsighted. II. Translate the following paragraph into English. Within the choroid, covering the posterior two-thirds of the eyeball, is the final destination of the light, the light-sensitive layer

18、called the retina. The purpose of everything in front of the retina is ultimately to form an appropriate real image of the outside world on this layer. This image, like that of a single thin lens, is inverted on the retina. The effective focal length of the relaxed eye is about 17mm, which means tha

19、t the retinal image is the same size as that which would be formed by a single 17mm focal length lens. When the eye is fully accommodated for nearby objects, the effective focal length shrinks to about 14mm. Stylistic Features 2: Syntactic features 1. tense and passive voice科技英语文体特征2：句法特征一、时态（tense）

20、由于科技文章侧重客观叙述科技事实，力求严谨和清楚，避免主观色彩和感情因素，因此具体动作发生的时间很少涉及，所以英语动词16个时态中，最常用的有5个，即一般现在时、现在进行时、一般过去时、一般将来时和现在完成时。其中最常使用一般现在时，其次是一般将来时和现在完成时。 1. 一般现在时1）叙述过程A scientist observes carefully, applies logical thoughts to his observations and tries to find relationships in data, etc.2）叙述客观事实或科学定理Sound travels thro

21、ugh the air in waves.A complete rotation of the Earth in relation to the pole star takes four minutes short of twenty four hours. Work is equal to the product of force and the distance through which the force moves.3）通常或者习惯发生的行为Alternating current is usually supplied to peoples houses at 50 cycles p

22、er second. 2. 一般将来时表示将来发生的行为或情况。In time, many things now unknown will become known.口语中常用的be going to 在科技英语中很少用到。3. 现在完成时表示到现在为止发生的行为或者已经发生的行为对现在有影响。During the past few years, several countries have pooled their resources in order to carry out certain types of scientific investigations more efficient

23、ly.Cast iron is more brittle than iron which has been heated but not melted, but it is easier to shape, as it can be poured into moulds. 二、 被动语态 （passive voice） 被动语态在科技文中使用得非常频繁，为什么？ 1. All the insulating substances were damaged by the sea wave.The heat loss can be considerably reduced by the use of

24、 firebricks, round the walls of the boiler. The spectrum of the substance could be determined by testing its effectiveness against various types of bacteria.Most of the drugs in current use were discovered by accident of trial and error.2. In the digital computer the numbers to be manipulated are re

25、presented by sequences of digits which are first recorded in suitable code, then converted into positive and negative electrical impulses, and stored in electrical or magnetic registers. More examples:1. Computers have been widely used in our national defense.2. Finally, the application of this meth

26、od is illustrated with an example.3. A new computer-aided design method is presented in this paper.4. Many mathematical techniques have been presented in this chapter for use by the control system engineer. Starting with complex-variable theory, we then developed the Fouier transform And Laplace transform. The transform function, block d