Specter

“ When the sun is in the right position, an ordinary-sized man on a lower mountain will see a gigantic shadow of himself thrown upon a cloud beyond the Brocken, though it appears to be on the mountain itself, and it is so perfect a representation that it is difficult to believe it is only a shadow. But it can be easily proved. If the man stoops to pick up anything, down goes the spectre; if he raises his hand, so does the spectre; if he takes a step of two feet, the spectre takes one of miles; if he raises his hat, the spectre politely returns his salute. ”

“ We now come to another very important discovery, made by means of our prism and narrow slit—the determination of the composition of the photosphere or mass of luminous vapour surrounding the body of the sun.
A simple experiment will show how this brilliant discovery was arrived at. The light of a candle or other flame containing incandescent solid matter is passed through the spectroscope, and is found to decompose into a continuous spectrum, uninterrupted by dark lines. Between the light and the slit a spirit-lamp is placed, but no difference in the appearance of the spectrum is perceived. ”

“ The spectre takes the shape of a human figure, or, if the visitors are more than one, then the spectres multiply; they arrange themselves on the blue ground of the sky, or the dark ground of any clouds that may be in the right quarter, or perhaps they are strongly relieved against a curtain of rock, at a distance of some miles, and always exhibiting gigantic proportions. At first, from the distance and the colossal size, every spectator supposes the appearance to be quite independent of himself. ”

“ Or rather, the spectroscope in alliance with the photographic camera; for the spectral changes indicating the direction and velocity of motion in the line of sight can be systematically studied, as a rule, only when registered on sensitive plates. The upshot has been to bring within the cognisance of science the marvellous systems known as “spectroscopic binaries.” They are of great variety. Some consist of a bright, others of a bright and dark, pair. Those that revolve in a plane nearly coinciding with our line of vision undergo mutual occultations. ”

“ If the slit in a spectroscope is lighted with a yellow light from such a flame, and if a grating normal to the direction of the rays is substituted for the prism, then in the telescope there is seen a yellow image of the slit, and on each side of it one, two, three or more yellow images. If sunlight is used the central image is white, since all the colours are here assembled. The other images become spectra because the different colours are unequally refracted. ”

“ In the dead of night there was a noise as of iron, and, if you listened more closely, a clanking of chains was heard, first of all from a distance, and afterward hard by. Presently a spectre used to appear, an ancient man sinking with emaciation and squalor, with a long beard and bristly hair, wearing shackles on his legs and fetters on his hands, and shaking them. ”

“ It is no doubt important to recognize that the two types of spectra seem to represent two extreme cases of one formula, the significant difference being that in the line spectrum the distance between lines diminishes as we recede from the head, while in the case of the band it increases, at any rate to begin with. But, on the other hand, no one pretends to have found the rigorous expression for the law, and the appropriate approximation may take quite different forms when constants which are large in one case are small in the other. ”

“ Though we felt rather surprised at this discovery, we simply noted it down on our paper, and proceeded to verify it by chemical analysis.
The phantom obligingly allowed us to fill a small phial with the luminous matter, which Harry immediately proceeded to test with all the resources at our disposal. For purposes of comparison I filled a corresponding phial with air from another part of the room, which I subjected to precisely similar tests. At the end of half an hour we had completed our examination—the spectre meanwhile watching us with mingled curiosity and amusement ”

“ She therefore sought the advice of a man skilled in such matters, who directed her, whenever she saw a spectre, to say, ‘Go to Heaven;’ but when she met a hearse, ‘Hang on.’ Happening some time after to meet a hearse, she, through lapse of memory, cried out, ‘Go to Heaven,’ and straightway the hearse rose in the air, and vanished. Afterwards, meeting a spectre, she said to it, ‘Hang on,’ when the spectre clung round her neck, hung on her back, and drove her down into the earth before it. ”

“ Yet Wollaston years after passed his beam of light through a slit instead of a round hole, and the lines of the solar spectrum were for the first time visible. It was still left to Fraüenhoffer and others to interpret their meaning, and to-day men of science are able to speak of the elements which enter into the composition of distant worlds, and you or I by the aid of the spectroscope could detect infallibly the presence of blood in solution, in amount so small that our microscopes would be useless in the search. To take an illustration which more nearly concerns us to day. ”

“ SPECTRES OF THE LIVING (Tamhasg) .
Some thirty years ago a man in Tiree, nicknamed the Poult (am Big-ein) , was haunted for several months by the spectre of the person with whom he was at the time at service. The phantom came regularly every evening for him, and if its call was disregarded it gave him next evening a severe thrashing. According to the man’s own account, the spectre sometimes spoke, and, when he understood what it said, gave good advice. ”

“ But it is not so much in new lists of suns or satellites—though the name of these alone has, indeed, been legion—as in the fresh light cast upon the nature and constitution of older ones, that our age has been most singularly successful. The invention of the spectroscope, and the rapid development of spectrum analysis, have placed in the hands of astronomers a method and an instrument inferior in value only to the telescope itself. ”

“ But then it may be asked, How comes it that the prism and the spectroscope are so useful to astronomers? In answer we may say, that if we knew no more about the black lines in the spectra of the sun and stars than we knew forty years ago, the spectroscope ought still to be an astronomical instrument, because it is our duty to observe every fact in nature, even if we cannot explain it. ”

“ With improvements in the methods of experiment and research the number of lines has increased to hundreds and even thousands. The light from a glowing [Pg 50] solid or liquid element forms, on the other hand, a continuous spectrum, i.e. a spectrum which has no dark lines. An illustration of the solar spectrum with the strongest Fraunhofer lines is given at the end of the book.
In contrast to the solar spectrum with dark lines on a bright background are the so-called line spectra, which consist of bright lines on a dark background. ”

“ These variations in spectra depend upon the fact that any substance that is heated sufficiently to emit light whether in the heavens or on the earth, will give a spectrum. If it be a solid or liquid body, we shall have what is called a continuous spectrum—that is, a colored band bright from end to end, with no sign of any dark or bright lines about it. ”

“ Ordinary incandescent vapours from highly heated substances give discontinuous spectra, i.e. spectra in which the rays of coloured light are quite limited, and they appear in the spectroscope only as lines of the breadth of the slit. These are called line-spectra, and every chemical element possesses in the incandescent gaseous state its own characteristic lines of certain colour and certain refrangibility, by means of which that element can be recognised. ”

“ Let us use the term spectral index (s) to define a number expressing the spectral character of a star. Then we may conveniently define this conception in the following way. Let A0 correspond to the spectral index s = 0.0, F0 to s = +1.0, G0 to s = +2.0, K0 to s = +3.0 M0 to s = +4.0 and B0 to s = -1.0. Further, let A1, A2, A3, &c., have [Pg 18] the spectral indices +0.1, +0.2, +0.3, &c., and in like manner with the other intermediate sub-classes. Then it is evident that to all spectral classes between B0 and M there corresponds a certain spectral index s. ”

“ The spectroscopic analysis, when applied to the same problem, walks upon slippery ground and must take heed lest it also fall. The alleged displacement is a nice quantity, and instrumental sources of error have been pointed out which may explain away the whole of it. I lay no stress upon the large difference between Vogel and Huggins in the quantity of motion which spectrum analysis ascribes to Sirius, inasmuch as the direction of the motion is the same. We do not yet know all the elements which the earth contains. The spectroscope has already added four to the number. ”

“ The spectra produced by different dispersive media not only differ in length, but also in the breadth of the coloured spaces not being in the same ratio to each other; hence the spectra are said to be irrational, or the dispersion is said to be irrational.
Vision.—The visibility of an object depends upon the rays of light which emanate from each point of its surface presented to the eye being brought to a focus upon the ret ́ina or expansion of the nerve of sight lining the inside of the back of the eye ”

“ When a beam of light is passed through a hole of, let us say, the eighth of an inch in diameter and decomposed by a prism, the spectrum so produced is imperfect, inasmuch as an infinite number of spectra are thus superposed, and for this reason, that the rays of light entering on the right side of the aperture will give a spectrum falling in a different place to that formed by the rays entering on the left. In order, therefore, to diminish the confusion caused by the superposition of a number of spectra, the aperture ought to be reduced to a narrow slit. ”

“ But it was soon shown how spectrum analysis might be applied to the investigation of the sun and stars; and this result was reached through the solution of a problem which had been long an enigma to natural philosophers. The scope and conquest of this problem we must now endeavour to comprehend. A spectrum is pure in which the colours do not overlap each other. We purify the spectrum by making our beam narrow, and by augmenting the number of our prisms. When a pure spectrum of the sun has been obtained in this way, it is found to be furrowed by innumerable dark lines. ”

“ Violet-pole spectrum discussed. Most spectra have a general as well as special character.
B. The question of the violet-pole spectrum. Here I make the remark that in comparing other spectra with that of the Aurora, it is, I think, too much the practice to trust to the coincidence (more or less perfect) of one or perhaps two lines out of many; whereas we know by experience that most spectra have so well-marked a general as well as special character that, when once seen, they are recognized afterwards with the greatest ease and without measurements. ”

“ If I have a transparent body and allow light sent out by a body giving a continuous spectrum to fall through it, I often observe that the transparent body sifts out of the light falling through it certain kind of rays. Spectra are thus produced which are called absorption spectra, because the body which is under examination does not send out any light, but absorbs some vibrations which are made to pass through it. It is an important fact that a molecule absorbs just the rays which it is capable itself of sending out. ”

“ In essence a spectrograph consists of a narrow slit, one or two-thousandths of an inch wide, on which the star light is focussed. That passing through the slit falls on the collimator lens which makes it parallel and then on a prism or prisms, triangular shaped pieces of glass which change the direction of the light and decompose it, breaking it up into its constituent rainbow colours. The spectrum, as it is called, is focussed by a camera lens on a photographic plate or can be viewed by a small telescope if desired. ”

“ To produce such an effect it is only necessary to cause the solar image to move at a uniform rate across the first slit of the spectroscope and then, with the aid of a second slit, which takes the place of the eyepiece of the spectroscope, to isolate one of the lines, permitting the light from this line, and from no other portion of the spectrum, to pass through the second slit to a photographic plate. ”

“ In spectral analysis, these finders have a peculiar importance; for by means of them the precise point of the object under examination is known, to which the slit of the spectroscope in the principal telescope is directed. It is therefore necessary that the cross-wires, or in general the sights placed in the field of the finder, should correspond with great exactness with the slit of the spectral apparatus, and I had, of course, taken great care to secure this essential point. Special micrometers were also provided, to measure rapidly the height and angle of position of the protuberances. ”

“ With regard to the well-known statement that solids and liquids give continuous spectra, while gases give line-spectra, it must be remarked that metallic vapors show in nearly all cases a continuous spectrum before they condense. Oxygen gives a continuous spectrum at the lowest temperature at which it is luminous. Examining liquids and solids by the method of absorption, we find that many of them show discontinuous spectra, presenting fairly narrow bands. It is not denied that the nearness of molecules does not affect the spectrum. ”