Question

Who were the “other half” that Jacob Riis referred to in his 1890 book How the Other Half Lives?

Answer 1

Jacob Riis was referring to those that were living in squalid conditions (i.e. the poor)

Answer 2

Answer:  The poor who were living in awful conditions in New York City slums in the late 1800s.

Details:

Jacob Riis was a police reporter in New York.  In 1888, Riis took pictures of what life was like in city's slums.  Using his own photos as well as photos gathered from other photographers, Riis began to give lectures titled, "The Other Half: How It Lives and Dies in New York," in which he would show the pictures on a projection screen and describe for viewers what the situations were like.  He gave his lectures in New York City churches.  In 1989, a magazine article by Riis (based on his lectures) was published in Scribner's Magazine.  The book version was then published in 1890 as How the Other Half Lives: Studies among the Tenements of New York.

Riis blamed the poor living conditions on greed and neglect from society's wealthier classes, and called on society to remedy the situation as a moral obligation.