“Even if the use of the Hiroshima bomb was justifiable in order to precipitate an end to the war, the bomb dropped on Nagasaki three days later was clearly a test of new arms,” he wrote. “It cannot be justified.”
[…]

After the lecture, he toured a Los Alamos museum, where full-size models of the Hiroshima and Nagasaki bombs are on display. His wife, Akemi Shimomura, also a chemist by training and his longtime research collaborator, said that the Japanese government had been stupid to not surrender immediately after the Hiroshima bomb.

“Starting the war was stupid,” Dr. Shimomura replied.

The next day, they returned. Something was on his mind. The day before the Nagasaki bombing, Dr. Shimomura had seen a B-29 bomber drop three parachutes. The drop had puzzled him. He would later learn that they carried instruments for data transmission and measurement.

He asked John E. Pearson, the Los Alamos physicist who had invited him to lecture, about the instruments. After some hunting they found models of the original parachute payloads.

“Some guy came up and started explaining what we were looking at,” said Dr. Pearson. “Osamu said, ‘Yes. I watched them falling.’ I don’t think I’ve ever seen anyone quite as stunned as that guy.”

Atomic Bomb Survivor & Nobel Laureate Osamu Shimomura from “For Witness to Nagasaki, a Life Focused on Science – NYTimes.com”.