Complex sentence: After 1950, control of the firm passed to co-founder, Joseph Klingenstein, who was by all accounts a brilliant investor in stocks, and under his leadership the firm created one of the first professional research departments on Wall Street.
Split into simpler sentences: After 1950, control of the firm passed to co-founder, Joseph Klingenstein. He was a brilliant investor in stocks and under his leadership the firm created one of the first professional research departments on Wall Street.