Final answer:
The principal tool used in medical and biological research today is the microscope. Microscopes allow scientists to visually examine and study small biological organisms and molecules that are not visible to the human eye.
Step-by-step explanation:
The principal tool used in medical and biological research today is the microscope.
Microscopes are essential for visualizing and studying small biological organisms and molecules that are not visible to the human eye. They allow scientists to magnify and observe cells, tissues, bacteria, viruses, and other microscopic structures. By using microscopes, researchers can gain valuable insights into the intricate workings of living organisms and conduct detailed studies in the field of biology.