The picture shows a brass hand pump and a bell jar.
Courtesy of American Artifacts

A bell jar is used in biology to isolate a specimen from all other living things. Plath's father was a biologist, so she must have been familiar with this item.

Sylvia Plath felt isolated growing up in Wellesley, MA. and even more so, when she was a patient at McLean Hospital in Belmont, MA. There she was under constant observation and examination by men.