首页
登录
职称英语
The human criterion for perfect vision is 20/20 for reading the standard li
The human criterion for perfect vision is 20/20 for reading the standard li
游客
2024-05-30
1
管理
问题
The human criterion for perfect vision is 20/20 for reading the standard lines on a Snellen eye chart without a hitch. The score is determined by how well you read lines of letters of different sizes from 20 feet away. But being able to read the bottom line on the eye chart does not approximate perfection as far as other species are concerned. Most birds would consider us very visually handicapped. The hawk, for instance, has such sharp eyes that it can spot a dime on the sidewalk while perched on top of the Empire State Building. It can make fine visual distinctions because it is blessed with one million cones (视锥细胞) per square millimeter in its retina (视网膜). And in water, humans are farsighted, while the kingfisher, swooping down to spear fish, can see well in both the air and water because it is endowed with two foveae(凹窝)-areas of the eye, consisting mostly of cones, that provide visual distinctions. One fovea permits the bird, while in the air, to scan the water below with one eye at a time. This is called monocular vision. Once it hits the water, the other fovea joins in, allowing the kingfisher to focus both eyes, like binoculars, on its prey at the same time. A frog’s vision is distinguished by its ability to perceive things as a constant motion picture. Known as "bug detectors", a highly developed set of cells in a frog’s eyes responds mainly to moving objects. So, it is said that a frog sitting in a field of dead bugs wouldn’t see them as food and would starve.
The bee has a "compound" eye, which is used for navigation. It has 15,000 facets that divide what it sees into a pattern of dots, or mosaic. With this kind of vision, the bee sees the sun only as a single dot, a constant point of reference. Thus, the eye is a superb navigational instrument that constantly measures the angle of its line of flight in relation to the sun. A bee’s eye also gauges flight speed. And if that is not enough to leave our 20/20 "perfect vision" paling into insignificance, the bee is capable of seeing something we can’t ultraviolet light. Thus, what humans consider to be "perfect vision" is in fact rather limited when we look at other species.
However, there is still much to be said for the human eye. Of all the mammals, only humans and some primates can enjoy the pleasures of color vision. [br] The Snellen eye chart measures one’s eyesight by ______.
选项
A、the number of lines he reads
B、the number of letter he reads
C、the distance he stands away from the chart
D、the speed at which he recognizes the letters
答案
A
解析
首段第2句中的lines of letters和第3句中的bottom line表明视力的好坏是依靠看到视力表的行数来决定的,由此可确定A为本题答案。
转载请注明原文地址:https://tihaiku.com/zcyy/3611582.html
相关试题推荐
[originaltext]W:Therewasanewquizshowontelevisionlastnight,butwewer
[originaltext]W:Therewasanewquizshowontelevisionlastnight,butwewer
[originaltext]W:Therewasanewquizshowontelevisionlastnight,butwewer
[originaltext]W:Therewasanewquizshowontelevisionlastnight,butwewer
[originaltext]W:Therewasanewquizshowontelevisionlastnight,butwewer
[originaltext]EversincePhiloT.Farnsworthassembledthefirsttelevision
[originaltext]M:IjustfinishedreadingabookofshortstorybyHemlenGarlen
[originaltext]M:IjustfinishedreadingabookofshortstorybyHemlenGarlen
Readingbetweenthelines,you’llbeableto__________(觉察到作者对受害者深切的同情).sense
Thetruthisthatradiohasnotbeeneclipsedbytelevisionandcableandth
随机试题
主张高校课程应注重反映各学科领域的基本结构,高校教学应着重传授各门学科特有的基本
在居住区管线布置时,下列电力电缆与电信电缆的布置原则何者正确?()A.电力电
关于妊娠期母体的变化,正确的是A.每分钟肺通气量减少 B.血浆白蛋白增加 C
反复呕吐最可能继发的情况是:()A.低氯高钾碱中毒 B.低氯低钾碱
计算机教室中有一台学生机出现故障( )。A交换机故障 B代理服务器故障 C
下列哪一项属于循环系统的不良反应A.困倦 B.皮疹 C.气短 D.腹泻
1.2010年3月29日至30日,全国对口支援新疆工作会议在北京召开。国
下列关于证券公司从事自营业务禁止行为的说法,错误的是()。A:操纵证券市场给投资
A.硝酸甘油 B.呋塞米 C.硝酸异山梨酯 D.盐酸地尔硫 E.利血平作
A.扁平丘疹 B.淋巴细胞浸润带 C.白色条纹 D.皮肤 E.免疫异常是
最新回复
(
0
)