This fixature of the last impression on the living retina is by no means an accidental discovery, but is the final step in a laborious series of delicate researches. Nor is it the triumph of one man alone, the preliminary work having been performed by two distinguished physiologists. Prof. Boll of Rome discovered that the external layer of the retina in all living animals has a purple color, which is destroyed by light. During life the color is perpetually restored by darkness, but after death, Boll thought, it disappeared entirely. Prof. Kühne followed up this wonderful discovery and confirmed it in general, while correcting some of Boll's conclusions. He first ascertained that death does not necessarily destroy the color, since a retina that is not exposed to white light, but is kept in a room lighted by a yellow sodium flame, retains this "vision purple" for twenty-four or twenty-eight hours, even though incipient decomposition may have set in. It is destroyed at the temperature of boiling water or by immersion in alcohol, glacial acetic acid, and strong solution of soda, but in strong ammonia, saturated solution of common salt, or glycerine, it remains undiminished for twenty-four hours. On testing the effect of different colored lights upon this "vision purple," he found that the most refrangible rays change it most, while red has hardly more effect than yellow light. The color is not so delicate as Boll supposed. A few moments' exposure to daylight does not bleach the retina. This requires exposure for a considerable time to direct sunlight. The source of the color was found to be the inner surface of the choroid upon which the retina lies. If a portion of the retina is disengaged from the choroid and raised up, it bleaches, though the remainder, still attached portion, retains its color. If the raised flap is carefully replaced upon the choroid, it regains its purple hue. This restoration is believed to be a function of the living choroid, and probably of the retinal epithelium, though it is independent of the black pigment which this epithelium contains. This vision purple is the latest discovery in optical physiology, and it cannot fail to be a most important one. How far it will alter the received views upon the subject of changes in the strength of vision, which are now attributed to alterations in the distance of the crystalline lens, cannot be foretold. But it may be found possible to stimulate by drugs the restorative action of the choroid, and thus by gaining increased "definition," improve weak sight. As to the detection of murderers by photographing the last retinal picture from their victims' eyes, while these discoveries do not leave this an impossibility, they do not much improve the probability of its ever being done. Very often the sight of the assassin is not the last which comes within the victim's vision. Too long a time also usually elapses before discovery. These and similar difficulties must prevent the utilization of these discoveries in this direction, even if they should prove to be in themselves all that is hoped. The retinal picture has not yet been photographed, but it seems probable, from the above recounted experiments, that it can be.