We have here an explanation of the physical and chemical phenomena produced by the ethereal rays. A few vibrations of this medium, probably, would produce no perceptible effect on a mass of matter; but these movements are repeated hundreds of trillions of times in a second, and however feeble their influence at first, isochronism may finally give it great power. Let us consider, first, the molecules, which have some connection between them, as yet unknown, but probably only allowing a certain set of vibratory velocities, (as a cord will only vibrate so as to produce a definite series of musical notes.) If, then, these are isochronous with those of the surrounding ether, the movement of the latter will be communicated to the molecules; or, according to the new theory of heat, the body will be warmed. These movements may even become so violent as to permanently modify the manner of union of the molecules—that is, to change the state of the body from solid to liquid or gaseous; and, by this change of state, the molecules may become insensible to the vibrations which previously affected them; for the set which they can now perform may have been entirely altered. The phenomena of heat are then well accounted for by this theory. To explain similarly the chemical ones, we have only to suppose ethereal vibrations, such that the movement affects the atoms separately, instead of the whole molecule, so that, after they have been sufficiently prolonged, the connection between the atoms will be destroyed. According to this, the chemical action of light should always be one of decomposition; it is so undoubtedly in most cases, and in the rest, where a combination is produced—as, for instance, in the formation of chlorhydric acid by the action of the violet rays on a mixture of chlorine and hydrogen—we shall adduce hereafter some facts which explain them, and show that even here the real action of the rays is a decomposing one. It may be remarked that the introduction of these ethereal vibrations, whose dimensions and velocities are well known, into the region, still so mysterious, of atoms and of molecules, promises to lead to results long unhoped for. If, for example, the theory above stated is correct, it would appear that the union of the atoms is such that their necessary time of oscillation is shorter than that of the molecules; since the red rays, which have the greatest heating power, vibrate more slowly than the violet, which are the most active chemically, as stated some distance back.
The luminous action of the rays is no doubt the most important for us, but also the most difficult to study; we have, however, something to say about it, for real progress has lately been made in this department. In the first place, since we are speaking of sensations, it is necessary to notice that this subject has two very different parts, one of which belongs to natural science, and the other to psychology. We shall here speak only of the first, that is, of three classes of phenomena which are produced at the exterior extremities of the nervous fibres, on the line of the fibres, and in the brain respectively. It has been said, in a previous paper, that each of these requires a certain time, and the experimental results as to these times were there given. But this is all, or almost all, the knowledge, unfortunately, which we yet have as to what takes place in the brain. The conjecture has been made that the different kinds of sensations are due to different modifications of the cerebral extremities of the various nerves; or that at the interior extremity of the optic nerve, a different action occurs from that at the nerve of hearing, which seems probable, since there are good reasons for believing that the action of the main body of the nerve itself is precisely the same for all the sensations. In more than one way, our nervous system would then resemble the telegraph. All the wires are traversed by similar currents, but the registering apparatus is different in each. In one, the dispatch is read off upon a dial; in another, it is printed on a moving band; in a third, a facsimile is given of it, etc. The sending is also accomplished by different means; but in all cases the same agent, the electric current, is employed.
Since we are treating of the sensation of sight only in connection with the external vibrations, we need here only discuss the first of the three classes of phenomena mentioned above, those which correspond to the transmission of the dispatch. In explaining this, we shall follow the celebrated professor of Heidelberg, M. Helmholtz.
The use of the spectroscope, and the analysis of light as now made in physics, chemistry, and astronomy, might induce the idea that color is an intrinsic property of the rays, depending entirely upon the length of the undulation in each, and inseparably connected with it; but this is not the case. Color is an organic phenomenon, only produced in the living animal; and, in one sense, is very independent of the length of the wave, since it can even exist without the presence of any luminous ray. Its laws are admirably exhibited in a figure called Newton's circle. This circle has been modified by recent experiments, and has received three enlargements, which make it a sort of triangle with rounded corners; but it is very well to preserve its name, for, as yet, the claims of Newton in optics have not been contested in any "Commercium epistolicum." Let us briefly describe this figure. The red, green, and blue of the spectrum occupy the three corners respectively. Passing round the circumference, we go from red to green through yellow, from green to blue through greenish blue, and from blue to red through violet and purple. If we draw a straight line from any point of the circumference to the centre, we find the same color on all points of the line, but more and more diluted, so that the centre itself is perfectly white. This figure contains all possible shades of color, and has the following remarkable property, established by experiment. If we wish to know what color will be produced by the mixture of any others, we have only to mark upon this figure the points where the several colors are found, and place weights there proportional to the intensities in which the different colors are to be used in the combination; at the centre of gravity of these weights, that is, at the point on which the circle (supposed itself to be without weight) would balance when thus loaded, we shall find the resulting shade. This point does not need to be found by experiment, being more easily calculated mathematically.
Now it is evident from this that color is a mere matter of sensation; for it is obvious that the same centre of gravity can be obtained by an infinity of arrangements of the original colors, notwithstanding the diversity of their wave-lengths; and it will also be found that these various mixed rays, though having precisely the same color—that of the centre of gravity—will differ entirely in their other properties. They act variously upon the thermometer and on the sensitive photographic plate, and give different tinges to colored objects which they illumine. But upon the retina the action of all is the same. How is this result to be explained? We will answer without stating the proofs, which the limits of this article would forbid.
From what has been said, it will be seen that all colors can be produced by the mixture of the three fundamental or primary ones, red, green, and blue, which were placed at the three rounded corners of Newton's circle. It will also be supposed that, as in the theory of Thomas Young, nervous fibres of three kinds are found at every point of the retina. When these are excited in any way, whether by the vibrations of the ether, by lateral pressure on the ball of the eye, by a feeble electric current, or by any other means, they transmit the excitement to the brain; but the red fibres, (so to speak,) if they should act alone, would only produce, however they were irritated, the uniform sensation of a red such as we hardly ever actually see, more saturated than the ordinary red, and which would be found in our figure at the extreme summit of the rounded corner. The two other kinds of fibres would, of course, act similarly, producing colors more pure than are usually seen; since, in our usual sensations, the three are always mixed, each predominating in its turn; and this is the case even in the spectrum itself. The effect of the pure colors in the latter may, however, be heightened as follows: Let us fix our eyes, for instance, for a few moments on the blue-green. This is the complementary of the red. The fatigue will produce a momentary insensibility in the fibres corresponding to the blue and green, and, turning the eyes to the red part of the spectrum, the slight admixture of these colors there present will fail to excite sensibly the corresponding nerves, so that the red will be seen for a few seconds in great purity. But to return. The stimulus of the first set of fibres, though found more or less in all parts of the spectrum, will predominate at the red end, where the vibrations are slowest; that of the second set in the middle, where the green is found; that of the third, at the blue extremity. Why these inequalities? Why, also, do the dark rays, preceding the red and following the violet, fail to act on the retina? No certain reason can be assigned, but there are two very plausible ones: first, the media which the rays have to traverse in the eye before reaching the nerves have, like all other transparent bodies, the power of absorbing the vibrations, not all uniformly, but some in preference to others. This elective absorption would destroy or diminish the effect of the rays on the nervous fibres. The second reason, as will readily be surmised, is the want of isochronism between the vibrations of the rays and those of the nervous fibres.
In confirmation of this theory, a remarkable anatomical fact, noticed among many birds and reptiles, may be cited. These actually have in the retina three kinds of fibres: the first terminated by a small, oily red drop, the second by a yellow one, while the third have no perceptible appendage. Evidently, the red rays will arrive most purely at the first, the central rays of the spectrum at the second, while the blue and violet ones will act freely only on the third. It must be granted that no such thing has been observed in man and the other mammalia; but something similar may be found in the singular pathological phenomenon to which the chemist Dalton has given his name. Daltonism is most frequently an inability to perceive red. For eyes thus affected, the chromatic triangle or circle just mentioned is considerably simplified; but sad mistakes are the consequence. "All the differences of color," says Helmholtz, "appear to them as mixtures of blue and green, which last they call yellow." This disorder would be, according to the above theory, a paralysis of the first, or red fibres. The simplicity of this explanation is certainly in favor of the theory which gives it. But we had determined not to bring up arguments. Let us, then, pass on; remarking, however, one respect in which the eye, otherwise so superior to the rest of the senses, is inferior to the ear. Sounds, though combined to any extent in harmonies or discords, can readily be separated by an experienced ear. The eye, on the other hand, only sees the result of mixed colors; it needs instruments to rival the ear; and it is only by means of the prism that it can separate and classify the various vibrations which reach it.
But, provided with this prism, or spectroscope, it has lately done wonders. It has discovered and measured a whole world of new phenomena, which, according to the theory just developed, must be attributed to reciprocal exchanges of movement between the ether and the ponderable molecules. The light given by these has disclosed to us many secrets of chemistry, and especially of astronomy.
Before specifying the most recent of these discoveries, we will profit by what has already been said to explain very briefly the fundamental principles of spectral analysis. Transparent bodies, whether solid, liquid, or gaseous, exercise upon the rays an absorption which is called elective, because some undulations are allowed to pass, while others are stopped, according to their velocities; and one of the effects of this absorption is the color of such bodies. This is to be explained by the principle of isochronism. Those vibrations which, for want of it, cannot be imparted to the surrounding matter, pass freely; the others are absorbed. But it is remarkable that gases and vapors only absorb a small number of them, while solids and liquids retain a great many. Thus, supposing that we have obtained, in any way, a continuous spectrum—that is, one with no breaks—containing all the known rays, not only the visible ones between the red and violet, but also the rest outside of these limits, a liquid or solid body intercepting this light will entirely destroy, or considerably weaken, large portions of this spectrum; whereas a gas or vapor generally will only efface a few small ones, whose absence is detected in the luminous part of the spectrum by the dark, transverse lines which have been so long known in that of the sun. This is certainly quite extraordinary, since it would suggest the inference that in gaseous bodies, the molecules, though less condensed, or further from each other, than in solids or liquids, have a much smaller range of possible vibrations. Besides this, the researches of Mr. Frankland on flames have lately shown that, even in gases, this range increases as the density augments. These results must undoubtedly be considered as strange; but what, after all, do we know of the connection of the elements of matter? Without dwelling further on this point, we will mention the most important fact learned by these experiments: that this elective absorption is a complete test of the chemical composition of gases. In given conditions of temperature and pressure, each gas is perfectly distinguished from all others by the special absorption which it exercises upon the luminous rays. The principle by which chemical analysis is performed spectroscopically is thus evident. To find if any particular gas is to be found on the path of the ray, it is only necessary to develop the latter into a spectrum, and to see, by the position of the particular dark lines produced in it, if the absorption due to this gas has been effected.