Popular Science Monthly/Volume 23/May 1883/Lengthening the Visible Spectrum
| LENGTHENING THE VISIBLE SPECTRUM. |
By JOHANNES GÖTZ.
THE phenomena of refraction and dispersion teach us that a body in a state of intensest heat emits not alone powerful thermal rays, but also all possible sorts of light (luminous colors). Diffraction convinces us that radiation is a wave-motion of an extremely fine, elastic, fluid medium, ether, and at the same time it enables us to compute the wave-length of the single rays. As is known, our apparatus suffices for taking cognizance of from one hundred and sixty to seven hundred and ninety billion undulations of heat or light per second, while all the vibrations of the ether lying either below or above are withdrawn from our direct observation. It is the purpose of this article to show what ways and means have been found for rendering the latter rays at least partly visible to our eye.
We throw a spectrum upon a white screen by means of a prism. The rays of the inferior number of undulations (ultra-red) will lie beyond the red, the superior (ultra-violet) beyond the violet end of the spectrum. We will begin with the latter.
We replace the screen, generally covered with ordinary white paper, by another one, the covering of which is impregnated with silver chloride, a combination of the two elements, chlorine and silver.[1] When the light has for some time operated upon this preparation, we interrupt it, and examine the screen by the light of a candle. We find that the coating has become blackened; that the blackening is insignificant at the place where we formerly had red light, but that it increases the nearer we approach toward the violet end; that it finally attains its maximum beyond this place, and gradually grows weaker until, at a certain distance, it disappears from the violet end. Whence this blackening? By the operation of the ether-waves, the combination of chlorine and silver was dissolved, the chlorine passed into the air in the form of vapor, while the silver was precipitated in microscopically fine pearls upon the paper. The coating became black, because silver is not metallically lustrous in such minute division, but simply constitutes a black powder.
This experiment convinces us that rays will still be found beyond the violet end of the spectrum, which, on account of their high number of undulations, are shrouded from our sight, and yet betray their presence by the decomposition of silver preparations. These rays have been called actinic or chemical rays, and their spectrum the chemical spectrum.
Our proposition is, "May not these rays, even if only partially, be rendered visible to the eye?" Let us, for this purpose, shut out all the other vividly luminous colors of the spectrum, so that they shall not interfere, by their excess of illumination, with the feeble effect of the ultra-violet rays. We interpose a black screen in their path, and cut off all except the extreme violet ones. The ultra-violet rays now become visible upon the white screen;[2] we see them of a feebly lustrous lavender-gray.
This method of rendering the ultra-violet rays visible is extremely primitive. Stokes, the successor of the great Newton in the professor's chair of the University of Cambridge, indicated a means by which our object is attained much more effectively. He introduced a piece of calcic fluoride into the ultra-violet part of the spectrum, and found that this crystal began to shine brightly with a blue light. Before we attempt to elucidate this peculiarity, however, let us consider the influence of the motion of ether upon a body.
When a ray arrives upon the surface of a body, three things may be imagined: The ray is either reflected, or it is transmitted, or absorbed. We describe the first two cases as reflection and refraction. In the third case, the ray is absorbed, and serves for heating the body, which itself emits again the arriving motion of ether in the form of calorific rays.[3] Besides these three cases, another, a fourth one, is possible, to wit, that, although the arriving rays are absorbed, they are not wholly employed in the heating of the body, but are partly altered into rays of another number of waves, and are emitted again under their changed form. Stokes, who first investigated this alteration more minutely, named it fluorescence.
Investigations demonstrate that, besides the calcic fluoride, there are an entire series of fluid substances possessing this property of conversion: for instance, petroleum; again, the solution of the highly esteemed febrifuge—quinine sulphate; esculine, an extract of the bark of the common horse-chestnut; leaf-green, or chlorophyl; eosine, frequently used in the manufacture of red ink; and, finally, in a high degree, the solution of a substance discovered by Bayer, of Munich, fluoresceine (resorcinphtaline).
Let us get better acquainted with these fluorescing substances. Best for this purpose is a narrow glass tube, filled with rarefied air—a so-called Geissler's tube, surrounded by an envelope, containing solutions of such substances in four divisions (Fig. 1).
By means of the electric current we bring the inclosed air to a red heat. It emits whitish-violet light, which penetrates into the fluorescing solutions, and is by them partly transmitted and partly absorbed. We find the transmitted light to be colored differently by the different fluids, but that these latter themselves begin to shine in different colors—for instance, eosine, green; quinine sulphate, blue. By the former, 
Fig. 1. the transmitted light is red; by the latter, almost white. In both cases, consequently, the rays emanating from the red-hot gas were screened as it were, and the retained part converted into rays of another number of undulations, to wit, into green with eosine, and blue with quinine.
Far more intense and admirable in color become the appearances, when we make the powerful rays of the electric light parallel by means of a condensing lens, pass them through a square glass vessel filled with pure water, and to this add the substances by drops. We will begin with eosine. We pour a little of the solution into the water, and an admirable, vividly green-colored cloud at once spreads within the vessel. If we place a white screen behind the vessel, we find that the transmitted light appears red upon it. Eosine, consequently, possesses the property of only permitting the red rays to pass, and of altering them into green light, while absorbing all the others. We take fresh water, and repeat the experiment with fluoresceine. The green of the generated cloud now is far more vivid, while the transmitted light is yellow. We close the experiments with quinine sulphate. The cloud is colored delicately blue, but the transmitted light is pure white. Which rays were absorbed in this case? A later experiment will answer the question.
There is an occurrence very generally found in nature which is dissimilar in form, but analogous in essence, with the fluorescence. With fluorescent substances, the emission of light ceases as soon as illumination is interrupted. If light is thrown upon calcium preparations, a part of the rays is also absorbed, and altered into rays of another number of waves. But these preparations emit the absorbed rays partly only after the cessation of illumination. Owing to the weakness of the light emitted, it becomes visible only after the preparations have been placed in darkness. Since this peculiarity of subsequent illumination is analogous to the development of light occurring when a piece of phosphorus is rubbed in darkness, it has been called phosphorescence. Both the duration and intensity of this subsequent light depend upon the nature of the substances employed. There are those known, the light of which disappears very quickly after its emission, and again those by which the illumination lasts as long as eighteen hours—of course, while growing constantly feebler.
For the study of this phosphorescence we again make use of a Geissler's tube, the exterior envelope of which contains four phosphorescent substances in four divisions (Fig. 2). By means of the electric current, we raise the temperature of the inclosed air to a glow-heat, and cause the emitted rays of light to operate for about a minute upon calcium salts.
After interrupting the light, the salts appear in four different colors, to wit: orange, yellow, green, and blue. This property of phosphorescence is universally found in nature. I call to mind the glowing of decayed wood, that of fire-bugs, etc. Various inferior organisms are provided with special glands for secreting phosphorescent substance. During excitement this is exuded by the animal, and begins to emit light. The phosphorescent light of the tropical waters is produced by myriads of minute organisms, by which the substance is secreted.
An Englishman, Balmain, succeeded some time ago in manufacturing a substance of a fairly intense and durable phosphorescence. It is used for painting watch-dials, match-boxes, door-signs, etc., to make them self-illuminating. In tenor with the nature of things, these articles can discharge their functions only after they have previously been exposed to the light of day, or some other energetic source of light.
We have in this manner become acquainted with means of altering rays of one number of waves into those of another number, and we will employ these means of rendering the ultra-violet rays visible to the eye. For this purpose, we must seek for substances possessing the property of absorbing these ultra-red rays, in order to emit them as rays of an inferior number of undulations. Besides the calcic fluoride, the above-named solutions of quinine sulphate and of esculine will answer our purpose.
We again throw a spectrum in the above-described manner, and introduce a calcic fluoride crystal into the ultra-violet part. It begins at once to shine vividly with a blue light. A writing with cyanuret of barium and platinum upon white paper is invisible in ordinary white light, but, as soon as we expose it in the ultra-violet end of the spectrum, it emits greenish-blue light. Finally, if we throw the spectrum upon a screen, the paper covering of which is saturated with quinine sulphate, we shall at once observe that it extends largely beyond the violet end. The ultra-violet rays now begin to appear with a pale-blue color. By the operation upon the quinine sulphate, therefore, the invisible rays have been converted into visible, illuminating ones.
Deeply violet-colored glass possesses the property of transmitting only the extreme violet and ultra-violet rays, and absorbing all the others. We cause a pencil of white luminous rays to emanate from the incandescent carbon-points of an electric light, which, rendered parallel through a condensing lens, passes through a square glass vessel containing clear water. We introduce a pane of violet glass into its path. The illuminating radiation is cut off by it, and only a few violet rays are transmitted. The ultra-violet ones, however, are represented much more abundantly in the now invisible pencil. Their existence is revealed at once when we add a few drops of the quinine solution to the water. Bright blue-colored clouds now move within the vessel, generated by the quinine absorbing the ultra-violet rays and changing them into blue light. The appearance becomes still brighter by substituting the more energetic fluorescent esculine in place of the quinine.
If we draw a sketch[4] upon yellow paper with an esculine solution, it is invisible in daylight as well as by electric light. But if we insert the violet glass into the pencil-cone, the single parts of the picture begin to shine vividly with a blue light. The sketch flames up at once in the obscurity before our eye, and we might imagine that we have been transported into Fairy-land.
We have until now had our attention engaged with the ultra-violet rays; it remains to speak of their practical adaptation. On account of their chemical effect upon the salts of silver, they constitute the basis of an important branch of industry—photography. As we have seen above, the red rays have almost no influence upon such preparations, while the effect of the yellow and green ones, when compared to that of the blue, violet, and ultra-violet, is not very great. Many mysteries of photography, incomprehensible to the layman, are explained hereby. A red and a black dress, for instance, are exactly alike upon a photograph, while blue and white, in their effect, approach nearer to white.
We now turn to the opposite end of the spectrum—the ultra-red rays. Our proposition is, "Are we able to render perceptible to the eye, the organ of sight, those rays that operate upon our sense of feeling simply as conveyers of heat?" We can attain our purpose only by augmenting the number of vibrations the thermal rays, by their influence upon suitable bodies, in such a manner that they are rendered perceptible to the visual organ. We provide the electric lamp with a parabolic reflector, A B, silvered and polished within, with incandescent carbon-points in its focus. The intense rays of the lamp are made parallel by the reflector, and pass through the room as a bright horizontal column. We recognize their course by the illuminated dust-particles of the air. We interpose another spherical reflector, C D, also silvered and polished, in the course of the rays. According to the law of reflection, all the rays falling upon the latter unite into one point, the focus (Fig. 3). It is easily recognized, since it brightly illuminates the dust-particles of the air. But not alone the luminous, but also the thermal rays, are united at this point. We become convinced of this fact by holding a cigar at the focus: it is at once ignited, begins to smoke, and bursts into flame. In consequence of the concentration of 
Fig. 3. the caloric rays, the most varied inflammable bodies may be ignited at this luminous point. Paper is perforated and charred in a moment, zinc consumes with a bright violet flame. Very thin, blackened platinum is brought to a white heat, and emits an intense white light. We place a test-tube filled with water within the focus; it begins at once to burst into bubbles, and commences to boil. Are these observed occurrences effected by the rays of heat or of light, emanating from the incandescent carbon-points? We answer this question by placing a body in the course of the rays, which, although it transmits the luminous rays, absorbs the thermal ones. Such a one is a concentrated solution of alum in water. We place a glass vessel, filled with this perfectly transparent solution, between the two reflectors, and in this manner sift the rays emanating from the carbon-points. The luminous focus is still there, but the ebullition of the water in the test-tube ceases at once. We remove the vessel, and ebullition is resumed with violence. Those rays, therefore, that caused the boiling were absorbed by the alum solution. This had meanwhile been raised in temperature, and, if left sufficiently long, it would begin to boil. We return the solution into the path of the rays, and place white paper within the focus. It is illumined brightly, but not consumed. We repeat the experiment with gun-cotton wrapped in white paper. It might lie there for a hundred years without exploding. We remove the vessel, and explosion occurs at once. We continue the experiment with black paper, by bringing it into the focus of the rays sifted through the solution, when it is at once perforated and ignited. Gun-cotton wrapped in black paper explodes almost instantly. Why is it that the same rays that left white paper intact at once ignite black? The luminous radiation transmitted by the solution is not absorbed, but reflected, by the white paper. It is brightly illumined, but not heated. Black paper, however, absorbs these rays, is heated thereby, and ignites.
The preceding experiments convince us that the combustion and heating of bodies in the focus are solely caused by the dark rays emitted by the carbon-points. We confirm this conviction by introducing into the path of the rays a body transmitting the dark radiation with the greatest facility, while completely absorbing the luminous one. According to Tyndall's experiments, this condition is complied with to a very high degree by a solution of iodine in carbonic disulphide. We introduce a very thin-walled glass cell, filled with such a solution, between the reflectors. Light is now cut off, but heat passes through freely. The focus is absolutely dark, but it still contains heat, of which fact we can soon convince ourselves by introducing a cigar into it: it is ignited and bursts into flame. White as well as black paper is charred and ignites in it. A piece of platinum-foil is raised to white heat in the dark focus. If we examine the incandescent platinum with a prism, we find that it emits again all colors, from the most extreme red to the most extreme violet; consequently, we have here the counterpart of fluorescence. The dark rays, by the augmentation of the number of vibrations, are converted into luminous ones, influencing the eye. Tyndall, who first observed and examined this appearance, called the conversion calorescence.
We have thus passed through a domain of physics, the more exact knowledge of which we mainly owe to the researches of our century, more especially to very recent times. That part of radiation perceptible to our organs of sense was extended far beyond the violet end of the spectrum, in investigating the chemical effects of light and fluorescence. We succeeded at the same time in rendering visible that part simply felt by the eye. It can not for a moment be supposed that there are no more rapid or slow rays, besides those already known to us, and ranging in the number of vibrations from one hundred and sixty to two thousand billions. Their existence can just as little be doubted as that of the ultra-violet. Whether we shall ever succeed in rendering them perceptible to our organs of sense remains a task for the investigations of the future.—Westermann's Monatshefte.
- ↑ This and most of the following experiments succeed well only with a very great power of light. It is necessary, therefore, to sustain the lamp by a battery of from sixty to eighty elements. In the present case, sixty-four large Bunsen elements were employed.
- ↑ In this experiment, the screen, impregnated with silver chloride, was replaced by a white one.
- ↑ The name of calorescence would be far more applicable to this peculiarity than to the one mentioned further below.
- ↑ The one employed by Mr. W. Fried, of Augsburg, represents a Renaissance ornamentation, of about sixty centimetres in diameter.