How did the ultramicroscope help scientists study Brownian motion?

The ultramicroscope was instrumental in the study of Brownian motion because it allowed scientists to visualize and measure the movement of extremely small colloidal particles that were invisible under ordinary light microscopes.12

By illuminating these particles with a strong light beam directed perpendicularly to the microscope's optical axis, the instrument caused the particles to scatter light, appearing as flashes against a dark background. Because these colloidal particles are smaller and exhibit greater activity than larger particles, they provided a clearer, more easily measurable subject for observing the random, oscillatory fluctuations characteristic of Brownian motion.12

This capability was critical for verifying the quantitative theories of Brownian motion proposed by Albert Einstein and others. Specifically, the French physicist Jean Perrin utilized the ultramicroscope to conduct the measurements that successfully confirmed these theories, providing empirical evidence for the existence of atoms and molecules as physical entities. For this groundbreaking work, Perrin was awarded the Nobel Prize for Physics in 1926.21