Explanation:

- William Crookes was a notable physical chemist and physicist known for his work with cathode rays and spectroscopy.

- He discovered and named the element thallium in 1861, using the spectroscope.

- Plutonium is a synthetic element discovered by scientists at the University of California, Berkeley, in 1940.

- Germanium was discovered by Clemens Winkler in 1886.

- Beryllium was discovered by Nicolas-Louis Vauquelin in 1798.

Thallium is the correct answer.