prints the image onto paper. Because the
image is transferred twice, the words and
pictures on the plate do not have to be
reversed.
History
By the AD 700s people in China were
using carved wooden blocks to make
prints. They applied ink to the block
and then pressed the block onto paper.
This technique was used to make the
Diamond Sutra in 868. It is the first
known printed book.
In the 1400s in Germany, Johannes
Gutenberg invented a printing press that
could make copies of whole pages of
text. Gutenberg arranged type (metal
blocks with raised, reversed letters on
them) on a tray. He rubbed ink on the
raised surface of the type. He then used
the machine to press a sheet of paper
against the inky type. Gutenberg’s
invention could easily print many copies
of a page—and even copies of a whole
book.
The printing press created a communication
revolution. Before this time,
people copied books by hand. It was a
slow process, so there were few books
available. After the invention of the
printing press, more books could be
printed and sold. Printing also led to the
creation of newspapers.
#More to explore
Book and Bookmaking
• Communication • Gutenberg,
Johannes • Newspaper
Prism
A prism is a piece of glass or other seethrough
material that has several flat
sides, called faces. Many prisms have
three long faces in between two triangleshaped
ends. However, prisms can be
made in many forms and shapes. People
use prisms to bend light.
Light travels through space in the form
of waves. Scientists use an idea called
wavelength to describe these waves.
Some light waves have long wavelengths,
while others have short wavelengths.
Each wavelength produces a different
color of light.
Usually all the wavelengths of light
blend together to form white light. But
when light enters a prism, it bends.
Light waves of different wavelengths
bend by different amounts. The longest
wavelengths bend the least. The shortest
wavelengths bend the most. This
bending separates the light waves so
each color can be seen. This band of
colors—red, orange, yellow, green, blue,
indigo, and violet—is called a
spectrum.
A prism shows that white light is actually a
combination of many colors.
BRITANNICA STUDENT ENCYCLOPEDIA Prism 153
Prisms may be found in binoculars,
microscopes, and many other instruments.
They help to bend the light that
enters these instruments toward the
viewer’s eyes. Scientific instruments
called spectroscopes use prisms to separate
the energy given off by light
sources, such as stars. Each chemical
that makes up a star produces a different
wavelength of light. Scientists can measure
the different wavelengths to tell
what the stars are made of.
..More to explore
Light
Prison
Prisons are buildings that hold people
who have been convicted, or found
guilty, of serious crimes. Of all the
world’s countries, the United States and
China have the most known prisoners.
Purposes of Prisons
Prisons serve three main purposes. They
punish criminals by taking away their
freedom. They also keep dangerous
criminals off the streets. Finally, prisons
can help to rehabilitate prisoners, or
make them into better people. Prisons
do this by giving prisoners an education,
job training, or help in getting over the
problems that caused them to commit
crimes. These problems may include
emotional problems, mental problems,
or the abuse of drugs or alcohol.
Types of Prisons
Maximum-security prisons hold people
who have been convicted of the most
serious crimes—for example, murder or
hurting someone with a gun, a knife, or
another deadly weapon. Prisons built to
hold less dangerous criminals are called
minimum-security prisons. Jails are prisons
for people who committed small
crimes or people waiting for a trial.
There are separate prisons for men and
women.
History
Until the 1700s most prisons were not
used for punishment. Instead of prison,
common punishments included death
and banishment (sending a person out
of the country). Prisons mainly held
people who could not pay their bills,
people waiting for a trial, and guilty
people waiting to be given their sentences,
or punishments. By the 1900s,
however, most prisons held convicted
criminals.
The conditions in early prisons were
horrible. Disease, violence, and abuse
were common. In the late 1700s reformers
(people who work for change) in the
United States and Great Britain began to
look for ways to improve prisons. By the
1900s many prisons offered prisoners
Prisoners live in small rooms called cells. rewards for good behavior, education,
154 Prison BRITANNICA STUDENT ENCYCLOPEDIA
and job training. Reformers today continue
to work for safer and more effective
prisons around the world.
..More to explore
Crime
Pristina
Population
(2004 estimate)
165,840
Pristina is the capital of Kosovo. It is
located in the east-central part of the
country.
Pristina is a cultural center for ethnic
Albanians, who make up most of the
population of Kosovo. A museum in the
city teaches about the history and people
of the area. Southeast of the city is the
Gracanica Monastery. It was built in the
1300s and contains interesting frescoes,
or wall paintings. The monastery is a
