Primary Sources. Primary sources are the most useful items for learning about an historical event. Today we will look at some primary sources that provide clues about the reputation of the Metropolitan Police Force (Peelers) in the nineteenth century. Think about the following questions when you examine the sources: When was the source written/painted/photographed? Was it an official photograph/poster? What does the source want you to think? How reliable is it and what can we learn from the source?.