Kühne's original work falls into two main groups, the physiology of muscle and nerve, which occupied the earlier years of his life, and the chemistry of digestion, which he began to investigate while at Berlin with Virchow.[3] In 1876, he discovered the protein-digesting enzyme trypsin.[4]
He was also known for his research on vision and the chemical changes occurring in the retina under the influence of light. Using the "visual purple" (or rhodopsin), described by Franz Christian Boll in 1876, he attempted to make the basis of a photochemical theory of vision, but though he was able to establish its importance in connection with vision in light of low intensity, its absence from the retinal area of most distinct vision detracted from the completeness of the theory and precluded its general acceptance.[3] Kühne also pioneered the process of optography, the generation of an image from the retina of a rabbit by applying a chemical process to fix the state of the rhodopsin in the eye.[5]
He was elected member of the Royal Swedish Academy of Sciences in 1898