A kind of blue dye (颜料) called folium had been widely used to paint pictures on the pages of manuscripts (手稿) created in the Middle Ages. But long ago, this watercolor fell out of use.

"Manuscripts are an important part of the cultural works of art in the Middle Ages," said scientist Maria in an email to CNN. "Color is a basic feature of these manuscripts. To let ancient manuscripts. keep a good condition for future generations, we sometimes need to repaint their inside pictures through the ancient dye — like folium — that was once used in these manuscripts. But everyone who knew how to prepare folium had died long ago. So 'I led a team to study this dye."

Her team turned to books from the 1400s and found one that described the plant that was its source. It is a tiny plant with silvery-green leaves. Days later, the scientists reached a village in south Portugal, where nobody knew this little plant. However, after carefully going through a plenty of different plants growing there, the scientists finally raked out the plant they wanted and brought some back to their laboratory.

Back in the lab, the scientists produced the dye with the help of an ancient text on colors. The text has clearly described the ways to make it. "It's cool to have done that work of looking in the historical records and traveling back in time," says Casadio, who was not part of this study. It recreates an ancient process. By making the dye, the scientists could study its chemistry without experimenting on priceless delicate (易损的)  works of art, she points out.

Now, the researchers don't know the dye's stability(稳定性)and its ability against the environment that may wear and break it down over time. "To know such information is necessary if we desire to make these colors last long enough," Maria said. "So our study will continue."

When buying a backpack, there are several things you should consider. {#blank#}1{#/blank#}A backpack used for carrying school supplies, for example, may not need to have as many features as a backpack that is to be used for hiking or camping.

When buying a backpack，look for one that can easily be placed on your back. Many people carry their backpacks over only one shoulder.{#blank#}2{#/blank#}Backpacks are designed to be carried completely on your back, supported by both shoulders, in order to help evenly distribute (分配) the weight. In fact, carrying a backpack that is either too heavy or used improperly is one of the leading causes of back injuries in students.

{#blank#}3{#/blank#} If the backpack will be used for carrying textbooks, it must be wide enough to contain the width of the books it will carry. If the backpack is for hiking, it needs to be large enough to carry supplies such as a solar blanket and other camping things.

Another consideration when buying a backpack is the number of compartments (隔层) the backpack has. When used for hiking or camping, it's often helpful for a backpack to have several smaller compartments. {#blank#}4{#/blank#}

The material is also important when buying a backpack. When buying a backpack for hiking or camping, it is best to buy one that is strong enough. {#blank#}5{#/blank#} If you are buying a backpack for college use, it might also help prevent your books and homework from getting wet while you are walking from one building on campus to another in the rain.

A. But this is actually a mistake.

B. There are backpacks different from traditional ones.

C. It is helpful to buy a backpack with much space inside.

D. It is important to buy one that will be comfortable to carry.

E. This way, important items such as knives are easy to reach.

F. Thus, your clothing or supplies will not get damaged in bad weather.

G. The most important consideration when buying a backpack is its future use.