Read the excerpt from "Hokusai's The Great Wave” by Neil MacGregor.

Here’s a snatch of the letter from the president of the United States that Perry delivered to the Japanese emperor:

Many of the large ships-of-war destined to visit Japan have not yet arrived in these seas, and the undersigned, as an evidence of his friendly intentions, has brought but four of the smaller ones, designing, should it become necessary, to return to Edo in the ensuing spring with a much larger force.

But it is expected that the government of your imperial majesty will render such return unnecessary, by acceding at once to the very reasonable and pacific overtures contained in the president’s letter . . .

This was textbook gunboat diplomacy, and it worked. Japanese resistance melted, and very quickly the Japanese embraced the new economic model, becoming energetic players in the international markets they had been forced to join. They began to think differently about the sea that surrounded them, and their awareness of the possible opportunities in the world beyond grew fast.

What type of outside source does MacGregor use in this excerpt?

a college textbook
a newspaper article
a historical letter
a Japanese website