Please read the Second Revision instead of this article, which is retained only as the record of the processes of my solving the mystery.
The crucial point in the development of Hideki Yukawa's meson theory came to him one night in October 1934. He writes in his autobiography Tabibito  as follows:
My new insight was the realization that this distance [the effective range of the nuclear force] and the mass of the new particle that I was seeking are inversely related to each other. Why had I not noticed that before?
From the fact that the electron has the rest mass, we consider that the strength of the interaction decreases rapidly as the distance between the neutron and the proton becomes large compared with h/(2πmc).
One of the questions addressed to him was this: "We hear that the Japanese people study sitting on their legs in a straw-matted room. Did you, Dr. Yukawa, write your paper sitting on your legs or sitting on a chair at the Western-style desk?" Hideki thought a little while and said, "I did in neither of those ways. I put my thoughts together at night in bed." This is true. Hideki wrote the paper that brought him Nobel Prize after he had kept thinking many nights at the age of 27 in the year of Shōwa 9 [Note by the translator: 1934]. This story seems to have wrongly come across to Japan. Thus, in Japan they believe that the idea flashed to him in the middle of a night. [Translated by T.T. from Japanese.]
- H. Yukawa, Tabibito (The Traveler), translated by L. Brown and R. Yoshida (World Scientific, 1982) p. 202.
- H. Yukawa, Sūbutu-gakkaisi, Vol. 7, No. 2 (1933) quoted in Nihon-no Buturigaku-shi (History of Physics in Japan) (Tokai University Press, 1978) p. 319 (in Japanese; English translation of the quoted passage by the present author).
- M. Kawabe and M. Konuma, Butsuri Vol. 37, p. 265 (1982) (in Japanese).
- H. Yukawa, Atogaki (Afterwords) in Tabibito, (Kadokawa, 1960) (in Japanese).
- H. Yukawa, Hon-no Naka-no Sekai (The World in Books) (Iwanami, 1963) p. 182 (in Japanese).
- S. Yukawa, Kuraku-no-Sono (Kōdansha, Tokyo, 1976) pp. 349–350 (in Japanese).