USA > Maine > York County > Parsonsfield > A history of the first century of the town of Parsonsfield, Maine > Part 15
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LIGHT.
The phenomena of light are more striking and more easily made subject to observation and experiment than those of the other physical disciplines; and it is doubtless for this reason that the science of optics was early cultivated with success and reached a comparatively high development. The early philosophers regarded the eye itself as a source of visual rays, which, coming in contact with external objects, gave a knowledge of them by a process similar to that involved in touch. Some modifications of this view were here and there held. Thus Empedocles taught that, besides the rays which go out from the eye, others go out from visible objects and mingling with each other, the two sets produce images of the body seen. Such general doctrines prevailed down to the middle ages, though Aristotle held that there must be some medium between the eye and the object, by means of which vision is effected, just as is the case with respect to sound. In opposition to the prevailing view, Aristotle inquired, " If the eye be the source of light, how is it not possible to see in the dark ? "
Some facts concerning the reflection of light must have been known at an early date (Milton is no doubt right in making the first woman acquainted with it. See Par. Lost, Bk. iv, line 460), as is evident from Moses, and from the fact that mirrors are found in ancient Egyptian tombs. Archimedes is said to have set the Roman fleet on fire at Syracuse, by means of mirrors with which he con- centrated the rays of the sun upon their ships! We find, however, no statement of principles which implies that anything more had been attained than an em- pirical knowledge of the facts of reflection. The ancient knowledge concerning refraction was apparently even less exact than that concerning reflection, though the bending of a ray coming obliquely from an object lying in the water must have been known. The well-known experiment of the cup and the coin is men- tioned by Cleomedes (A. D. 50). Ptolemy left a work on optics, in which he
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discourses on vision, reflection and refraction. The laws pertaining to these matters, however, he did not know. Passing over a long interval, in which nothing of much value was accomplished for the science of optics, it may be mentioned that Vitellio, in the 13th century, attempted some of the more obvi- ous problems, such as the measurement of the amount of refraction which takes place when a ray passes from air into water obliquely, though it is true that Ptolemy had attempted the solution of some of them before him. Vitellio knew the fact that light is dispersed as well as refracted, and he applied his knowledge, with success, to the explanation of the rainbow.
Mirrors of glass as well as of metal were known at an early period, but at first they were not coated with metal, as at present. The first who speaks of their be- ing coated with metal is Vincenz V. Beauvais, 1240. Mirrors were coated with an amalgam of mercury and tin, as at present, first in the 14th century. More important by far, the same century saw the introduction of spectacles. They were invented by Salvino degli Armati, who died 1317.
In 1311, the brothers Theodorich wrote a work in which, although the exact law of refraction does not appear, the modern explanation of the rainbow is given with sufficient clearness. Murolycus, about 1575, investigated certain problems, and among them the round image of the sun, which is seen when its light is admit- ted into a darkened room through a very small hole. In 1589, Porta invented the camera obscura, and compared it with the eye. Kepler explained the defect of the eye called short-sightedness, and showed why it is corrected by the use of concave lenses. He also explained long-sightedness, and showed that it is reme- died by the use of convex lenses.
The invention of the telescope, in 1608, was made by Lippershey, and it marks, doubtless, a most important epoch in physical science. The results which followed were, as generally happens when any great invention is made, mani- fold. Researches in optics were stimulated, in order to its improvement, and most astounding achievements in astronomy were made, among the first of which was the discovery of Jupiter's moons, by Galileo.
The mathematical statement of the law of refraction was made by Snell, in Leyden, 1626, and thus it appears that the invention of the telescope, the theo- retically best construction of which would imply a knowledge of this law, was made empirically.
In 1611, De Dominis attempted to account for the colors which are produced when white light passes through a prism: " When white light passes through a prism, more or less of darkness, from the material of the prism, is mingled with it according as it passes through a longer or shorter path." Hence, according to him, the blue portion of the spectrum is turned toward the thicker part of the prism, since blue is darker than red.
In 1644, Mersenne constructed the first reflecting telescope, although Zucchi had previously made an observation suggestive of it. It is clear, on considera-
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tion, that the early notions concerning the nature of light, might easily lead to two general theories. It might be regarded as consisting of material particles escaping from the eye or from the luminous body, or it might consist in the mo- tion of some medium intervening between the eye and the visible object, as Aristotle, in some vague way, believed. Descartes held a doctrine, in some sense, midway between these two general views. According to him, light is not produced by the setting up of waves in the proper sense, nor by the motion of particles emitted by the luminous body. It consists in a pressure, momentarily exercised, between bodies which are shining, and this pressure is able to affect our eyes by its impulses.
Grimaldi (1618-63) made an observation which, at a later period, led to an uni- versal acceptance of the theory of undulations. He admitted a sunbeam into a darkened chamber through a very small hole. In the beam, at some distance from the hole, he set up a small rod and received its shadow on a screen. To his surprise, he found, instead of a single sharp shadow, a central shadow trav- ersed by several colored luminous bands, and he found on either side of the general shadow narrow luminous bands, alternating with dark ones. It was now, for the first time, clear that "light can shine around a corner." Grimaldi called the phenomenon " diffraction," a name which it still retains. Grimaldi was lead by these and other observations to regard light as consisting of wave motions in some medium not otherwise evident to our senses, and he compared these mo- tions to those which result when a stone is thrown into the water. He even at- tempted to explain the presence of the colors which the luminous stripes alluded to show, by supposing that the light medium is subjected to agitations, whose velocities differ.
The colors due to thin films, such as appear on the surface of steel during the process of tempering, due to oxidation, and such as are exhibited by soap bub- bles, were mentioned by Boyle, though he did not give any proper explanation of them. This was first done by Newton, who strenuously advocated the corpuscu- lar theory in opposition to the theory of Huyghens. Hooke espoused the theory of undulations, and a most bitter controversy between him and Newton resulted.
Newton's work on the solar spectrum began in 1666, but the results of his re- searches were not laid before the Royal Society till 1672. As is well known, he showed that sunlight is composed of an indefinite number of colors, whose com- bined effect is to produce white light. He was no doubt justified in thinking this the most imporant discovery which had thus far been made concerning the nature of light.
Deschales, born in 1674, discovered the so-called diffraction spectrum which is produced when a beam of light is reflected from a grating consisting of a great number of fine lines ruled on a plane surface of glass or metal.
In 1704 appeared Newton's Optics, a complete treatise on all the then known phenomena of light. In this work he clearly shows that the production of the
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prismatic spectrum is due to the unequal refraction of the different colors of the sunbeam. Moreover, he determined the indices of refraction of the several pri- mary colors into which he divided the spectrum. He was then able to show that the order of the colors of the rainbow and the width of the band conform to cal- culation. He turned his attention to the telescope in the hope of finding some means of correcting the colors with which it apparently invested objects seen through it. He concluded that it was impossible to construct an achromatic object glass, and that the only hope of the desired improvement lay in the use of reflectors. Dolland, however, satisfactorily solved the problem by the use of two kinds of glass differing in their indices of refraction.
About the middle of the 18th century, Bouguer commenced the development of photometry, or the measurement of the intensity of light. In 1779, Lam- bert's work, in which he set forth the principles of photometry, appeared, and in it the light received from the sun and moon are estimated and compared.
Galileo assumed the velocity of light to be indefinitely great. The Florentine Academicians endeavored to measure it, but without success. Roemer, in 1676, solved the problem by means of observations upon Jupiter's moons. Bradley also solved the problem by an entirely independent method, in 1728.
The phenomenon of double refraction presented by Iceland spar and by other crystals was accounted for by Huyghens on the supposition that the elasticity of such bodies differs in different directions, and subsequent researches have con- firmed this view.
The explanation also rested on the doctrine of undulations. Newton, consid- ering the behavior of the two beams into which a single beam of common light is di- vided on passing through Iceland spar, concluded that each of them had ac- quired two-sidedness; that is, that the two beams had acquired different proper- ties on their contiguous sides. Malus, in 1808, discovered that light which has suffered reflection from the surface of glass, at an angle of 55 degrees, possesses the same properties, and he termed such light " polarized light." This observa- tion led to the complete overthrow of the corpuscular theory of Newton, by Dr. Young, and Fresnel, who independently showed that the puzzling phenomena of diffraction could be explained by the assumption of undulations which can interfere so as to destroy or to reinforce one another. Moreover, they showed that the peculiar phenomena, which are alone due to polarized light, can only be explained on the theory of undulations which take place at right angles to the path of progress of the light. The act of polarization consists in throwing all the transverse vibrations or undulations into two sets of parallel planes at right angles to each other.
The hypothesis of Newton, besides being inconsistent with the phenomena of polarization, or, at least, incompetent to explain them, requires that the velocity of light shall be greater in dense bodies than in rare ones. It therefore be- came important to decide this question by experiment. Foucault undertook this
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work and decided in favor of the theory of undulations. At the same time, as the character of the experiment permitted, he determined the velocity of light independently of astronomical methods. The experiment consists in the use of a rapidly revolving mirror, from which the beam of light is sent to a distant mir- ror so placed as to send the beam back on its path by reflection, and in the use of some means of measuring the displacement of the beam from its point of de- parture, due to the motion of the revolving mirror. If we know the rate of rev- olution of the mirror, the distance to the fixed mirror, and the amount of the displacement of the beam, it is easy to calculate the velocity of light. More re- cent repetitions of the experiment, with various modifications, by several per- sons, have been made, among which may be mentioned those by Michaelson and Newcomb, in this country. The result may be stated in round numbers as 300,- 000,000 metres in a second.
Photography is an art of recent origin, and a short sketch of its rise and de- velopment must not be omitted. On its optical side, it had its germ in the inven- tion of the camera obscura, by Porta, in the second half of the sixteenth century. In the year 1556, Fabricius published an account of the printing of an image on a surface of chloride of silver (luna cornea) in shades of black and gray, by the action of sunlight, but he knew of no means of fixing the picture thus pro- duced. In 1777, Scheele found that the light in the blue and violet portions of the spectrum acted more strongly to change silver salts than the other portions. In 1802, Wedgwood and Sir H. Davy published a method of producing rough representations of objects by means of light, but even Davy knew of no means of rendering the results permanent. In 1813, Niepce, the elder, was able to re- produce engravings by the action of light, which was transmitted through them while they lay upon a plate of tin covered with a thin layer of bitumen of Judea, and also to fix in a transient way the image which was formed in the camera. Those portions of the bitumen which had been protected from the light remained unaltered, while the other portions on which the light acted were whitened. On plunging the plate thus changed into a bath of the essence of lavender, the unaltered portions were removed, and thus the picture was fixed. The result, however, was not satisfactory, and he tried many devices to im- prove it.
In 1826, Daguerre, who had made some unsatisfactory advances in the rising art, solicited a correspondence with Niepce relating to the subject. The result was a compact between the two to work jointly for the perfection of some plan of procedure. Daguerre abandoned the bitumen process, and sought to secure better results with iodide of silver. He found that with this he could produce a faint picture. This could be developed by the use of petroleum oil. At last he was led to try the vapor of mercury as a developer, when at once, as by magic, the picture was developed. He wrote to Niepce concerning his success, but Niepce died before learning of it.
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Photography was at last a fixed fact, and many now living will remember with what incredulity and curiosity the announcement was received.
Daguerre entered into partnership with the son of his deceased co-laborer, and endeavored to secure capital to work the invention, but with the usual success- failure. He then decided to give his invention to the State. This was done through the intervention or Arago. The optical appliances of Daguerre were little suited to the production of pictures of moving beings, and as the reproduc- tion of the human face was especially desirable, the opticians were stimulated to the production of better lenses, while, in the meantime, the perfecting of the chemical processes was pushed in every hopeful direction. Attempts were made in 1840 to secure portraits by the daguerreotype process, but the long time required to produce the picture made them unsatisfactory. Accellerators were found, such as the fumes of bromine, bromide of iodine, chloric acid, etc. These, together with the use of short focussed lenses, at last made it possible to reduce the sitting to four or five minutes in full sunlight.
In 1834, Talbot, in England, was trying to fix the image formed in the camera on paper. Herschel solved the problem by the use of hyposulphite of sodium. This was a most important step. Besides being important in itself as adding to the range of the art, it led to the perfecting of paper so as to render it suitable for the purpose. In 1839, Herschel used glass plates to support his sensitive films for the taking of negatives. Niepce de Saint-Victor, employed glass coated with a thin film of albumen, which he rendered sensitive by plunging it into a bath of potassium iodide, and subsequently into another of silver nitrate.
LeGray suggested the use of collodion instead of albumen as a means of hold- ing the sensitive silver salts. The collodion process, till lately, almost univer- sally employed in portrait and landscape photography, in its full and nearly per- fect development, was introduced in 1851 by Scott Archer and Dr. Diamond.
What has just been said applies to the process for producing the negative, or the picture in which the lights and shadows as they are in nature are reversed. The positive of the finished pictures is, as is well known, printed on paper. This is done by simply placing the negative flat on sensitive paper and exposing to the direct action of sunlight till the picture appears in full intensity, after which it is fixed by immersing in a bath of hyposulphite of sodium. It is then washed and " toned " with chloride of gold.
In the same year in which the discovery of Daguerre was announced Mungo Ponto found that paper, which has been treated to a solution of bichromate of potassium, can have a photographic image developed by the action of light in a camera or under a negative. This is due to the oxidizing action of the bichro- mate under the influence of light. It was soon found that other organic bodies besides paper-such as gelatine, gum, starch, albumen-are acted on in a similar way, and that, where the action takes place, they are rendered insoluble. Poitevin made use of these facts for the production of pictures with these substances,
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mixing with them powdered carbon. Swan, Woodbury, Johnson and others extended the process and improved it. Means were soon found, notably by Albert and Edwards, to print in printer's ink as many positives from such nega- tives as might be desired. For purposes of duplicating books, engravings, and illustrations, the process in some one of its modifications is invaluable, and is in constant use.
More recently the so-called wet process, in which collodion is employed in con- nection with a nitrate of silver bath for preparing the sensitive plate, have been largely superseded by the so-called dry plate processes. In these the bromide of silver is generally employed. The silver salt is introduced into the collodion, or, better, into gelatine, and the resulting emulsion is poured over the glass plate in an even film, in the dark room, and carefully dried with exclusion of light. Such plates, when carefully prepared, are exceedingly sensitive, and therefore rapid in their action. They are hence suited to instantaneous photography. With good light they require only a very small fraction of a second to produce good pictures. They are therefore used to produce pictures of objects in rapid motion, as the crowds and carriages in motion in the bustling street, birds in flight, horses in the race, etc. The most important application of these rapid processes is to be found in astronomy, where they can do the work of the observer with the greatest fidelity. Indeed they record the presence of objects which escape the eye alto- gether. They have the advantage over the eye that they do not get tired, and that plates may be exposed for long periods, and thus the effect of the faintest light is rendered cumulative. Photography has in fact opened a new method of attack for the astronomer, and brought within his reach a class of problems which otherwise were insoluble. This will be alluded to under our observations on the spectroscope.
Our limits will permit only the most brief mention of the principles of " Spec- trum Analysis." This is the name applied to a method of ascertaining the com- position of bodies by examination of the light which they emit when brought into a volatile condition by the application of heat. In 1802, Wallaston admitted a solar beam into a darkened chamber through a narrow slit and viewed it at a dis- tance of ten or twelve feet, through a colorless glass prism. He thus found the spectrum traversed transversely to its length, by a great number of dark lines and colored bands. In 1814-15, Fraunhofer published in the "memoirs " of the Academy of Munich a map of these dark lines as they appear in the solar spec- trum, and designated some of the principal ones by letters of the alphabet. These lines are hence known as Fraunhofer's lines. The spectroscope employed by him consisted of a glass prism to which the narrow beam of light was admitted, and a telescope through which the resulting spectrum was viewed. By means of this arrangement he was able to see and to map between five and six hundred lines. Kirchhoff modified the spectroscope of Fraunhofer by increasing the number of prisms and by the employment of a " collimator," that is, by the use of a second
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telescope in which the eye-piece was replaced by a tube bearing an adjustable slit through which the light was admitted. With this arrangement he made a map of more than three thousand lines. When it was attempted to compare the maps made by one observer with those made by another, it was found that, while the lines were in general arranged in the same order, their distances from each other depended on the variety of the glass or other material of which the prisms were composed, on the temperature at which the observations were made and of the peculiarities of adjustment. To avoid this inconvenience recourse was had to the spectrum produced by "gratings;" that is, to the spectrum which is formed when a beam of light formed by a narrow slit falls on a plate of glass having a great number of fine lines ruled very close to each other and at equal distances. Gratings answering the purpose better have more recently been made by ruling the lines on the surface of speculum metal, which is made quite plane. The spectrum is then produced by interference taking place in the reflected beam. The greatest accuracy is required in the ruling of the lines to secure that they shall be drawn at equal distances. Our own countrymen, Mr. Rutherford of New York, and Professor Rowland of Baltimore, have succeeded in the construction of divid- ing engines by means of which this result is secured to a very high degree.
Professor Angström, of Sweden, was among the first to employ a spectroscope in which the prism was replaced by a grating, in mapping the solar spectrum, on something like a complete scale, and his work will always remain a monument of skill and wonderful accuracy. It is, however, now completely superseded by the magnificent photographs of the spectrum recently made by Professor Rowland. In these there are clearly drawn by the action of the light itself many thousands of lines, which are at their normal distances depending on the absolute length of the light waves at their respective places in the spectrum. The gratings by means of which these wonderful spectra are produced are ruled on concave metallic sur- faces ground and polished with mathematical precision.
But what is the meaning of these lines? This question is easily answered in the light of a principle announced by Euler. It is as follows: the same kinds of undulations, or waves, can be received or absorbed by bodies as the bodies them- selves would be able to emit under the same conditions as those in which the luminous or light giving body is placed in any given case. Angstrom was led to the conclusion that any body at a glowing heat gives out the same kinds of light and heat as it will absorb when such rays fall upon it. With some limitations this principle has been established. The first decisive experimental proof of it was made by Kirchhoff in 1859. If the lime of the calcium light be viewed through the spectroscope, there will be seen a continuous spectrum with the red at one extremity and the violet at the other. But if now the flame of an alcohol lamp be placed before the slit so that the light from the incandescent lime must pass through it, and some common salt be introduced into the flame, there will be seen a dark line crossing the spectrum in the yellow. On cutting off the light
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from the lime a bright line will be seen at the exact point where the dark one was .. In other words the salted flame can absorb the same kind of light which it can emit. By the expression, the same light, is meant light having the same color or having the same wave length.
To explain the occurrence of the Fraunhofer lines we may consider that the sun is a body at so high a temperature that all known substances, at its sur- face, must exist in a condition of vapor. Thus the principal body of the sun must be surrounded by clouds of incandescent vaporized materials. These, though hot enough to be self-luminous cannot be at so high a temperature as the interior nucleus of the sun itself, since radiation must be constantly taking place. Thus, in accordance with the principle stated above, these vaporized matters absorb, in part, such peculiar kinds of the light, coming from the sun's nucleus as they would be able to emit if at the same temperature as the nucleus itself; hence the solar spectrum is traversed by great numbers of comparatively dark lines, corresponding in thickness to the width of the slit which admits the light to the prism or grating of the spectroscope.
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