when we are in a fairly dark room, or outside at night away from light, we can still see, but we can't see the colors of things very well. Why is that? This moment of Science is going to look at night vision.
there are two kinds of ligth-sensitive organs located in the backs of our eyes:rod-shaped and cone-shaped. Both rods and cones are sensitive to light.
The difference between them is that the rods allow us to see in very dim light but don't permit detection of color, while the cones let us see color but they don't work in dim light.
when it gets dark the cones lose their ability to respond to light.
The rods continue to respond to avaliable light, but since they cannot see color, so to speak, everything appears to be various shades of black and white and gray.
A curious thing is that in dim light you can see more clearly out of the side of your eyes, because the light-sensitive rods are more highly concentrated off to the side in the back of your eye.
So next time you're out on a clear night, notice how little color you can see, and how you can see objects like dim stars better out of the corner of your eye than from the center.
congruent,adj全等的，一致的，相称的
we have shown that the two triangles OAB and IAB are congruent.
They want to work in an organisation whose values are congruent with their own.
conjunction,n联合，结合
This conjunction of an immense military establishment and a large arms industry is unprecedented in the American history.
consign,v交付，委托
He consigned his business to his brother's care.
convivial,adj欢乐友好的
The three spent a convivial evening together.
cordial,adj热诚友好的
we are received a cordial welcome at the university.
courteous,adj礼貌地，客气的
The hotel staff are friendly and courteous.
covet,v贪求，渴望
He has long coveted the chance to work with a famous musician.
cowardice,n懦弱
He lost everything because of his cowardice.
credulous,adj轻信的，易受骗的
we are less credulous than we used to be.
cumbersome,adj笨重的
Although the machine looks cumbersome, it is actually easy to use.
cunning,adj灵巧的，精明的，狡猾的
It was a cunning piece of detective work.
He was as cunning as a fox. (getting what is wanted in a clever and often deceptive way)
debase,v贬低
we can sculpt our existence into something beautiful, or debase it into ugliness.
decadent,adj堕落的，衰败的
Don't let decadent ideals eat into yourselves.
deluge,n大量，泛滥 v 使充斥
A deluge of manuscripts began to arrive in the post.
we have been deluged with applications for the job.
demise,n死亡，终止
Smoking, rather than genetics, was the cause of his early demise.
demography,n人口统计，人口统计学
A combination of demography and the rising costs of America's huge entitlement programs will be starting to bankrupt the country.
demote,v使降级
when rating agencies demote a country's credit rating, it means they think it is now a riskier place to invest.
demur,v反对，提出异议
At first she demurred, but then finally agreed.
deprecate,v批评，反对， 贬低，轻视
He deprecated the low quality of entrants to the profession.他抨击了行业中的滥竽充数
The teacher should not deprecate his student's efforts.
derelict,adj被遗弃的 ，玩忽职守的
The rubbish consists of derelict spacecrafts dead satellites, pieces of metal ejected during collisions and dropped tools etc.
The officer was charged with being derelict in his duty.
descent,n下降，衰落, 血统，出身
An Airbus A300 crashed on its descent into Kathmandu airport.
Its citizens of African descent provided a special contribution in making New Orleans the birthplace of jazz.
diminutive,adj极小的，微小的
The doll's house contained diminutive furniture.
dire,adj可怕的，迫切的
The government looked as if it would split apart, with dire consequences for domestic peace.
The government is in dire need of reform.
disabuse,v使醒悟，使纠正错误
I did not disabuse them of the view that people in rural area like to please strangers.
disavow,v否认，拒绝
Dr.Samuels immediately disavowed the newspaper story.
discreet,adj谨慎的，考虑周全的
He was always very discreet about his love affairs.
discretion,n自由决定权，n，谨慎，考虑周全
School governors have the discretion to allow parents to withdraw pupils in exceptional circumstances.
Larsson sometimes joined in the fun, but with more discretion.(但显得更加矜持）
dissemble,v掩盖，伪装
Henry was not slow to dissemble when it served his purposes.
draconian,adj严厉的，残酷的
It's a draconian measure, but along with curfews, the injunction has quieted the streets considerably.
dull,adj枯燥无味的
The chairman leavened the dull meeting with a few jokes.