Luminous flux

“ How is it that the oxygen and hydrogen flame gives so little light, and with a solid body present, pours forth such a flood of brilliancy?
The production of artificial light by electrical and chemical agencies will necessarily find some consideration under their respective heads. There are numerous phenomena which connect themselves with luminous power, or appear to do so, which, in the present state of our knowledge, cannot come immediately under our attention. ”

“ On the other hand, some other objects, such as a lighted lamp, a burning coal, or a red hot iron, would be seen if no outside light were present. Such bodies are said to be luminous. Most luminous bodies are hot and become non-luminous on cooling. There are, however, some bodies that are luminous at ordinary room temperatures, as the firefly and some phosphorescent paints. When light emitted by a luminous body strikes an object, a portion of it is always reflected. ”

“ Two bright lights may be made to produce darkness.—If two pencils of light radiate from two spots very close to each other in such a manner that they cross each other at a given point, any object placed at that line of interference will be illuminated with the sum of the two luminous pencils. If we suppose those rays to move in waves, and the elevation of the wave to represent the maximum of luminous effect, then the two waves meeting, when they are both at the height of their undulation, will necessarily produce a spot of greater intensity. ”

“ There is absolutely no philosophical basis for distinction between the visible and invisible radiations of the sun, except in the one point of vibration-frequency—their pitch, to use the analogy of sound. The expressions thermal, luminous, and chemical rays are apt to be misleading. All the waves of solar radiation are carriers of energy, and when intercepted do work, producing heat, or vision, or chemical action, according to circumstances. ”

“ From the very brightness of the light of some of the fixed stars we may infer the intensity of that dark radiation, which is the precursor and inseparable associate of their luminous rays.
We thus find the luminous radiation appearing when the radiant body has attained a certain temperature; or, in other words, when the vibrating atoms of the body have attained a certain width of swing. In solid and molten bodies a certain amplitude cannot be surpassed without the introduction of periods of vibration, which provoke the sense of vision. ”

“ If a luminous body is surrounded by empty space, the light which it emits suffers no loss of energy as it travels outwards. The intensity of the light diminishes merely because the total energy, though unaltered, is distributed over a wider and wider surface as the rays diverge from the source. ”

“ According to the theory of emission, the transmission of light energy is effected by the actual transference of light-corpuscles from the luminous to the illuminated body. According to the theory of undulation there is a material medium which fills the space between the two bodies, and it is by the action of contiguous parts of this medium that the energy is passed on, from one portion to the next, till it reaches the illuminated body. The luminiferous medium is therefore, during the passage of light through it, a receptacle of energy. ”

“ An eventful feature in practical lighting is the proper scattering or diffusion of light, by shades, screens, and reflecting surfaces. We want to emulate the diffusion of daylight. It is marvelous how whitewashed surfaces do this. Well-selected globes act as though they were self-luminous; they scatter light and produce shadows. The power of the light, however, to penetrate fogs does not appear to be any greater than that of gaslight. ”

“ Those that exist beyond the red ray are capable of producing heat, while those that are beyond the violet excite chemical action. These invisible violet rays can be actually made perceptible to the eye, or, in other words, the undulations or waves proceeding from this end of the spectrum can be made to strike against certain substances and induce luminous vibrations, so as to connect the dark space beyond the violet with a brilliantly illuminated band. ”

“ In the emission from both of these sources we have a mixture of obscure and luminous rays; but the ratio of the latter to the former, in the lime light is greater than in the spiral; and, as the very meaning of transparency is perviousness to the luminous rays, the emission in which these rays are predominant must pass most freely through transparent substances. ”

“ Now, we may compare the emission of light-waves by a luminous object to the throwing of corks in our illustrative case. The rate of flow for light-waves is indeed infinitely faster than that of any river, being no less than 185,000 miles per second. The successive light-waves are set in motion at infinitely shorter time-intervals, since for extreme red light there are no less than 458,000,000,000,000 undulations per second, and for extreme violet no less than 727,000,000,000,000 ”

“ We reach the third stage, the lamp, with some seven per cent of the original energy still available. In this stage our only means of producing luminous energy is to heat the molecules of some substance, whereby we are compelled to waste the greater part of our efforts in producing heat, which is worse than useless, before we obtain the light rays. "Here, then, is the second unsolved problem, since even in the incandescent lamp and the arc lamp not more than from three to five per cent of the energy supplied is converted into light. ”

“ Outside of the colors we are accustomed to see, the solar spectrum includes other rays, some less refrangible than the red, which make themselves manifest by their calorific properties, and others more refrangible than the violet, which are remarked by their photographic effects, and by the action they exercise on fluorescent substances. The solar ultra-violet spectrum produced in the prism occupies an extent nearly equal to that of the luminous spectrum ”

“ The extreme richness of the electric light in invisible rays of low refrangibility was demonstrated, one-eighth only of its radiation consisting of luminous rays. The deadness of the optic nerve to those invisible rays was proved, and experiments were then added to show that the bright and the dark rays of a solid body, raised gradually to incandescence, are strengthened together; intense dark heat being an invariable accompaniment of intense white heat. A sun could not be formed, or a meteorite rendered luminous, on any other condition. ”

“ Responses to luminous stimulation occur in all cases equally well, whether the light employed be direct sunlight, diffused daylight, polarized light, or any of the luminous rays of the spectrum employed separately. On the other hand, neither the non-luminous rays beyond the red, nor those beyond the violet, appear to exert the smallest degree of stimulating effect. Hence, in all respects, the rudimentary eye of Sarsia appears to be affected by the same qualities of light as are our own. ”

“ Huyghens, on the contrary, supposes light to be caused by the waves or vibrations of an infinitely elastic medium—Ether—diffused through all space, which waves are propagated in every direction from the luminous body. In the first theory, a luminous particle is supposed actually to come from the sun to the earth; in the other, the sun only occasions a disturbance of the ether, which extends with great rapidity, in the same manner as a wave spreads itself over the surface of a lake. ”

“ Objects like chairs, tables, carpets, have no light within themselves and are visible only when they receive light from a luminous source and reflect that light. We know that these objects are not self-luminous, because they are not visible at night unless a lamp or gas is burning. When light from any luminous object falls upon books, desks, or dishes, it meets rough surfaces, and hence undergoes diffuse reflection, and is scattered irregularly in all directions. No matter where the eye is, some reflected rays enter it, and the various objects are clearly seen. ”

“ That the origin of the luminous column is to be sought for at its negative end; that the luminosity is an expression of a demand for negative electricity; and that the dark spaces are those regions where the negative terminal, whether metallic or gaseous, is capable of exerting sufficient influence to prevent such demand. ”

“ Nothing seems more natural than to suppose that the incident vibrations of the luminiferous ether produce vibratory movements among the ultimate molecules of sensitive substances, and that the molecules in return, swinging on their own account, produce vibrations in the luminous ether, and thus cause the sensation of light. ”

“ An analysis of the spectrum shows us that there are some changes regularly taking place in the state of the solar beam, which cannot be referred to the mere alteration of position. It may be inferred, from facts by long-continued observations, that the three classes of phenomena—light, heat, and chemical power, distinguished by the term Actinism—which we detect in the sun’s rays, are constantly changing their relative proportions. In spring, the chemical agency prevails; in summer, the luminous principle is the most powerful ”

“ If however the streams of the northern lights were supposed to rise above our atmosphere, they would only be visible at each extremity of their course; where they emerge from, or are again immerged into the atmosphere; but not in their journey through the vacuum; for the absence of electric light in a vacuum is sufficiently proved by the common experiment of shaking a barometer in the dark; the electricity, produced by the friction of the mercury in the glass at its top, is luminous if the barometer has a little air in it; but there is no light if the vacuum be complete. ”

“ Luminous phenomena may be produced by various means—chemical action is a source of light; and, under several circumstances in which the laws of affinity are strongly exerted, a very intense luminous effect is produced. Under this head all the phenomena of combustion are included. In the electric spark we have the development of light; and the arc which is formed between charcoal points at the poles of a powerful voltaic battery affords us the most intense artificial illumination with which we are acquainted. ”

“ If, instead of using the light of the sun, we form a spectrum with the light emitted by an incandescent gas, say hydrogen made luminous by the electric spark, we shall find that the spectrum consists only of a limited number of separate bright lines, of various colors. This shows that such a gas, instead of emitting light of all wave-lengths, as an incandescent solid body does, principally emits light of certain definite wave-lengths. ”

“ The proportion of the light or eye-affecting radiation to the dark heat in the total radiation from any source of light increases with the temperature, but it is not always merely a question of temperature. Thus the electric arc is hotter than a candle flame, and the sun is hotter than the electric arc. Hence, whilst the luminous rays only form 3 parts out of 100, or 3 per cent. ”

“ Whatever substances are here found in gaseous condition, the same will be evident by dark lines in the spectrum of sun or star, and the position of these dark lines will show, by coincidence with the position of the laboratory bright lines, all the substances that are vaporized in the atmospheres of the self-luminous bodies of the sky. ”

“ Although the luminous force acts only upon the nerve of the eye to stir up a sensation, yet we know how infinitely complex and varied is the world of color that results. There is evidence that the dark thermal and chemical radiations are of equal variability and complexity, yet there can be no doubt that all these multitudinous effects are due to a single mode of action. The difference between the thermal and the chemical rays is simply the difference between the red and the green ”

“ If a source of light has a luminous intensity of one candle in all directions, the illumination at a distance of one foot in any direction is said to be a foot-candle. This is the unit of illumination intensity. A lumen is the quantity of light which falls on one square foot if the intensity of illumination is one foot-candle. It is seen that the area of a sphere with a radius of one foot is 4p or 12.57 square feet; therefore, a light-source having a luminous intensity of one candle in all directions emits 12.57 lumens. ”

“ Instead of consisting simply of white light as a whole, it is now universally accepted that in this spectrum different properties belong to different parts. Light or luminous power to one portion; heat or calorific power to another; and chemical power or actinism to a third.
The visible solar or Newtonian luminous spectrum, resulting from the decomposition of white light by a prism, is only the middle portion of the whole solar spectrum. Beyond the red end there are rays possessing still greater-heating effect ”

“ A noticeable feature in these experiments is the small amount of physiological luminosity added by each successive candle to the first. Theoretically, if one candle equals 21 units intensity, two should equal 42, and three 63, whereas the physiological additions of luminosity are not only much less, but vary with different luminous intensities, as will be seen by the following comparisons:—
Chart 4 represents the Luminosity of gas from [53] a batswing burner reduced to one, two and three candle power at one foot distance. ”

“ All this time its luminous efficiency is increasing, because it is radiating not only an increasing percentage of visible radiant energy but an increasing amount of the most effective luminous energy. But even when it appears white, a large amount of the energy which it radiates is invisible infra-red and ultra-violet, which are ineffective in producing light, so at best the substance at this high temperature is inefficient as a light-producer. ”