There are certain words that appear in the subject headings of items in MERLIN that constitute primary sources. The most important of them is sources, but there are others.
In the simple keyword search box, you can put such words after SU: in order to specify the SUBJECT field.
ex.: british east india and SU:sources
In the advanced keyword search, you can put your search term(s) in the top box, and in the second box, put the word(s) you're using to locate primary sources in the next box, and change the drop-down menu to Subject:
Keywords that will help you find primary sources:
Depending on the period being studied, it can also be helpful to limit your search by publication date.
More keywords identifying primary sources in Library of Congress Subject Headings