Final answer:
Du Bois references John Brown in paragraphs 13 and 15 of the Address to the Niagara Movement to highlight Brown's rebellion against slavery as a symbol of resistance and to draw a connection between the struggles of the Niagara Movement and the fight against racism and oppression.
Step-by-step explanation:
Du Bois references John Brown in paragraphs 13 and 15 of the Address to the Niagara Movement because Brown's historic rebellion against slavery at Harper's Ferry served as a symbol of resistance and the fight for the rights of African Americans. By mentioning Brown, Du Bois is drawing a connection between the struggles of the Niagara Movement and the ongoing battle against racism and oppression. Brown's actions exemplify the kind of activism and determination that Du Bois and the Niagara Movement sought to embody.