First, Engelmann used a prism to disperse white light from the sun into the colors (wavelengths) of the visible spectrum.
Then, using a microscope, he illuminated a filament of green algae with the visible spectrum. The photosynthetic pigments in the alga absorbed some of the wavelengths of light, using the absorbed energy to drive the reactions of photosynthesis, including oxygen production.
Engelmann used his recently discovered aerotactic bacteria to determine which wavelengths of light caused the alga to photosynthesize most. Because the aerotactic bacteria were attracted to areas of highest oxygen concentration, they congregated around the regions of the alga that photosynthesized the most.
He then counted the bacteria associated with each region of the alga illuminated by the various colors of light.
Engelmann found that some wavelengths of light attracted more bacteria, suggesting that these wavelengths drive more photosynthesis than others.