One of the first fruits of the daguerreotypic art was the suggestion that unknown murderers could be detected by photographing the last image left on the retina of the murdered person's eye. The idea that this could be done seems to have taken strong hold of many imaginations, and we believe this suggestion is repeated to the police authorities of New York on the occurrence of every noticeable and mysterious murder. That such a detective task will ever be accomplished by photography is extremely doubtful, on account of the length of time that usually passes before the discovery of a murder. But science has now advanced so far that the image on the retina has been fixed and photographed. This has been done by Prof. Kühne of Heidelberg, but not with human subjects, as decapitation is one necessary part of the process. Prof. Kühne placed a rabbit four and a half feet from a closed window, in the shutter of which was an opening twelve inches square. The animal's head was first covered by a black cloth for five minutes and then exposed for three minutes. The head was then instantly cut off, and one eye taken out in a room illuminated by yellow light. The eyeball was opened and instantly plunged into a five per cent. solution of alum. This occupied two minutes, and the other eye, still remaining in the head, was then exposed at the window just as the first had been. It was then taken out and placed in the alum solution like its fellow. The next morning the two retinæ were carefully isolated, separated from the optic nerve, and turned. On a beautiful rose red ground a sharp image, somewhat more than one millimetre (one-twenty-fifth inch) square was found. The image on the first retina—that which was exposed during life—was somewhat reddish and not so sharply defined as that on the other.
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.
ACTION OF ORGANIC ACIDS ON MINERALS.
Dr. H. C. Bolton of the New York School of Mines has made the interesting discovery that minerals may be decomposed by boiling with organic acids, just as they are by treatment with the strong mineral acids. He has tried the action of such acids as citric, tartaric, oxalic, acetic, malic, and other acids, on finely powdered carbonates, silicates, sulphides, and other classes of mineral. All the carbonates examined (fourteen in number) dissolved with effervescence, sulphides were decomposed with evolution of sulphuretted hydrogen, and silicates with formation of gelatinous silica. This important discovery will greatly add to the resources of the mineralogist, who is compelled to do much of his work in the field. Hitherto he has been debarred from using the mineral acids (the action of which sometimes forms a decisive test) by the impossibility of carrying them in the pocket or wallet without danger. The organic acids are solid, and can be conveniently stowed away. Their action, however, is not so decided as that of the mineral acids, but this is not always a defect, but offers additional means of determination. For example, all the specimens of bornite and pyrrhotite examined yielded sulphuretted hydrogen with tartaric, citric, and oxalic acids, but chalcopyrite and pyrite do not. On the other hand, the use of the organic acids may give rise in some cases to the formation of nitric acid, which in its nascent condition will afford a very powerful agent of decomposition. Thus all the sulphides examined (seventeen), with the exception of molybdenite and cinnabar, were quickly attacked by citric or tartaric acid, to which a little potassium nitrate had been added. Potassium chlorate produces a similar though slower action. These examples are sufficient to show that Dr. Bolton has found a promising field of inquiry, and, singular to say, considering the attention which the action of organic acids has received, it is a field believed to be entirely new. He is continuing his researches.
SCIENTIFIC ORCHESTRATION.
Prof. Mayer has turned his valuable researches in acoustical science to æsthetic uses, and criticises the present mode of arranging orchestras, the defects of which he proves by experiment. He took an old silver watch, beating four times a second, and caused it to gain thirty seconds per hour, so that every two minutes its tick coincided with the tick of an ordinary spring balance American clock, also making four beats the second. The latter was placed several feet, and the watch two feet, from the ear. In this position the ticks of the watch were lost for nine seconds, about the time of coincidence. The tick of the watch disappeared, "with a sharp chirp, like a cricket's, and reappears with a sound like that made by a boy's marble falling upon others in his pocket." This experiment shows most effectively that one sonorous impression may overcome and obliterate another, but to do so it must be more intense and of lower pitch. If of higher pitch, it cannot neutralize the other sound, however much the first may exceed the latter in intensity. This discovery, Prof. Mayer thinks, is, "next after the demonstration of the fact that the ear is capable of analyzing compound musical sounds into their constituent or partial simple tones, the most important addition yet made to our knowledge of hearing." High sounds cannot obliterate low ones, but, on the contrary, the sensation of each partial tone of which compound musical sounds is formed is diminished by all the tones below it in pitch. These discoveries he applies to orchestration as follows: "In a large orchestra I have repeatedly witnessed the complete obliteration of all sounds from violins by the deeper and more intense sounds of the wind instruments, the double basses alone holding their own. I have also observed the sounds of the clarinets lose their peculiar quality of tone, and consequent charm, from the same cause. No doubt the conductor of the orchestra heard all his violins ranged as they always are, close around him, and did not perceive that his clarinets had lost that quality of tone on which the composer had relied for producing a special character of expression. The function of the conductor seems to be threefold: First, to regulate and fix the time. Second, to regulate the intensity of the sounds produced by individual instruments, for the purpose of expression. Third, to give the proper quality of tone or feeling to the whole sound of his orchestra, considered as a single instrument, by regulating the relative intensities of sounds produced by the various classes of instruments employed. Now this third function, the regulation of relative intensities, has hitherto been discharged through the judgment of the ears of a conductor, who is placed in the most disadvantageous position for judging by his ears. Surely he is not conducting for his own personal gratification, but for the gratification of his audience, whose ears stand in very different relations from his own in respect to their distance from the various instruments in action. Is it not time that he should pay more attention to his third function, and place himself in the position occupied by an average hearer? This position would be elevated, and somewhere in the midst of the audience. That the position at present occupied by the conductor of an orchestra has often allowed him to deprive his audience of some of the most delicate and touching qualities of orchestral and concerted vocal music, I have no doubt, and I firmly believe that when he changes his position in the manner now proposed, the audience will have some of that enjoyment which he has too long kept to himself." These views were verified by Prof. Mayer visiting different parts of the house during a public performance, and observing the different effects of the music. It is not to be supposed that a satisfactory change can be made at once. A quantitative analysis of the compound tones of all musical instruments must be made. On this work he is now engaged. One noteworthy result of his researches is the opinion that orchestral instruments should be made on different principles from those used in solos. The reason for this is, that certain over tones should predominate in orchestral instruments in order to give them their due expression in the midst of graver sounds. These exaggerated peculiarities will unfit them to be played alone. If the learned Professor's views are carried out, a theatre or opera manager will be obliged to own the instruments of his orchestra, and perhaps to have different sets for different musical works!
THE NITROGEN OF PLANTS.