Mantaro Hashimoto

The English sources I found are mostly academic, and interestingly, they often quote the Japanese linguistics scholar Mantaro Hashimoto, whose work dates back to 1939! He explained his methodology which included this text:

I started my study immediately after their arrival in Japan, which I had been expecting. During four months I recorded about 6,000 colloquial words and phrases, and noted carefully the type of articulation of each morpheme. Mr. Wui lived in my house for two months, July and August of 1959, and showed an unflagging spirit of co-operation. During this period I gathered about 300 

pages of colloquial sentences, forming a manuscript "Materials for the Study of Hainanese (海 南 語 料)". I was able to make clear the basic structure of colloquial Hainanese in its sound system, its vocaculary and its grammar. 

In my learning journey, I feel bittersweet that many existing literature about the Hainan language quote 2 non-Chinese persons, Carl C. Jeremiassen and  Mantaro Hashimoto.