Trichromatic color vision is the ability of humans and some other animals to see different colors, mediated by interactions among three types of color-sensing cone cells. The "trichromatic color theory" began in the 18th century, when Thomas Young proposed that color vision was a result of three different photoreceptor cells. From the middle of the 19th century, in his "Treatise on Physiological Optics", Hermann von Helmholtz later expanded on Young's ideas using color-matching experiments which showed that people with normal vision needed three wavelengths to create the normal range of colors. Physiological evidence for trichromatic theory was later given by Gunnar Svaetichin (1956).