Q: Given a scientific passage and an answer, generate a question for the given answer.
Passage: Scientists did not actually see viruses for the first time until the 1930s. That's when the electron microscope was invented. In 1915, English bacteriologist Frederick Twort discovered bacteriophage , the viruses that attack bacteria. He noticed tiny clear spots within bacterial colonies, and hypothesized that something was killing the bacteria. The tobacco mosaic virus shown in Figure below was the first one to be seen. Answer: electron microscope
A:
The invention of what instrument in the 1930s allowed scientists to see viruses for the first time?