Eyeglasses correct many different types of sight problems, including nearsightedness, farsightedness or astigmatism (散光). Trying to understand those different numbers on an eyeglass prescription (验光单) requires understanding the measurements optometrists (验光师) use when fitting you for corrective lenses (镜片).

Diopters (屈光度)
The numbers on your prescription are given in diopters, whose short form is "D". It is used to measure the focusing power, or amount of correction.
O.S. and O.D.
Optometrists use the Latin short form O.S. and O.D. to distinguish between your eyes. O.S. refers to your left eye, while O.D. refers to the right eye.
Generally, your prescription will show a number for O.S. and O.D. (given in diopters). The farther this number is on a number line from zero, the more correction your eyeglasses should provide. A plus sign means the eye is farsighted, while a minus sign means the eye is nearsighted.
Spherical Value
This plus or minus number for each eye is called the spherical value, or the degree of nearsightedness or farsightedness you have. Its short form is "S."
Cylinder
If you have an astigmatism, the optometrist will also measure its degree. The higher the number (given in diopters) marked as the cylinder of astigmatism, (its short form is "C"), the more astigmatism there is in that eye.
Axis
An astigmatism is also measured in terms of its axis, which is a number between 0 and 180. The axis of the astigmatism does not relate to the amount of cylinder, just the location of the irregularity of the astigmatism on that particular eye.
According to the first paragraph, various sight problems ______.

A．should be understood by the patients

B．can be avoided by wearing corrective lenses

C．should be tested by different optometrists

D．can be corrected by wearing eyeglasses

If you need a pair of corrective lenses, the optometrist will first value the numbers of ______.

A．Diopters

B．Spherical Value

C．Cylinder

D．Axis

From the prescription above we can learn that Li Hua ______.

A．is farsighted	B．is nearsighted
C．has no astigmatism	D．is normal in sight

The text is probably taken from ______.

A．an education report	B．a school bulletin
C．a local newspaper	D．an encyclopedia (百科全书)