How did Theodor Engelmann determine which colors of light were optimal for driving photosynthesis?

Engelmann used a prism to split light into its component colors and shined this light on green algal filaments. After adding oxygen-requiring bacteria, he observed where the bacteria congregated. He made the assumption that bacteria would congregate wherever oxygen concentration was high, and that high oxygen concentrations would coincide with areas of active photosynthesis. The bacteria clustered near blue and red light, suggesting that these wavelengths of light produced optimal photosynthetic conditions.