Report of the Eclipse Expedition to Caroline Island, May 1883. (1884) / Chapter Skim
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Report of Prof C S Hastings
Report of Prof C S Hastings
Pages 109-132
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From page 109... ...
S . H o L D E N : D E A R S I R : The late date at which it was definitively determined that the writer should be a member of the Eclipse Expedition rendered the preparation of the apparatus a laborious as well as a most hui-ried one The task would, however, have been far more difficult had not the trustees of the Johns Hopkins University generously placed all the facilities of the physical laboratory and the services of the university mechanic at my command.
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From page 110... ...
1 ) F = focal length of collimator objective, A = angular aperture of same, a = angular aperture desired, -- / = focal length of concave lens, u = distance from concave lens to slit, V =distance from concave lens to image of slit, and L = total length of collimator, the solution is as follows:
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From page 111... ...
The objectives of the collimator and viewing telescope have focal lengths of 12 8 inches and apertures of 1.6 inches, the available aperture of the single 60° flint-glass prism being also 1.6 inches. From the above dimensions it 18 evident that the effective aperture for central pencils was but 0.9 inch; but as the slit was made nearly an inch in length, and used radially, this was as much as could be employed under the condition that the whole pencil from nearly every portion of the slit should be transmitted.
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From page 112... ...
The ofiice of these prisms was to form two virtual images, each being that of one half the slit, separated at their nearest ends by an interval equivalent to the diameter of the solar image in the equatorial used The construction will be rendered evident by the diagram (Fig 2) , which represents a section of the apparatus bv a plane passing through tTie Sht and the line of colliuiation.
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From page 113... ...
This was, of course, lust at the middle of my spectrum band, along a line extending from red to violet, and they were confined to that side of the line corresponding to the eastern limb of the sun. I t is possible that some or all of these lines extended away from the middle of the band as very faint bright lines, but I could not be sure of the fact after the eclipse from a recollection of the hasty examination whicii I gave it.
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From page 114... ...
The bright lines of the chromosphere and corona stopped abruptly at the moon's limb, although the diftusion here should have been more sensible than towards the outer corona, because at the place where the change in,illumination was most rapid. I t will be recognized that my examination of this point was a careful one, for by it I assured mjself of the adjustment of the apparatus in the beginning of the observations.
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From page 115... ...
Now, I shall show that the biightness of the spectrum in the instrument under discussion is absolutely independent of the angular aperture of the " condensing lens." I t follows, then, that the use of the lens is not to " condense " light upon the slit I t seems that a much better name for this lens is one which describes its office at once, namely, "image lens" This name I shall venture to use to replace the old term in the following discussion: Let A = eflectiN e aperture of image lens. F = focal length of image lens.
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From page 116... ...
I n the second case the light is such as to yield a continuous spectrum. Then its brightness is proportional to the brightness of the sht-image and to the width of the slit, and inversely proportional to the .angular dispersion of the prism or its substitute.
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From page 117... ...
Suppose, for example, that the bright lines are so faint that in order to be distinctly visible they must not be less than^ 10'm width. If no telescope were used with the prisms, a form which has often been employed, this would render all but the strongest lines of-the solar spectrum invisible, if on the other hand a dispersive member of a tenth of the power but tenfold the aperture the same apparent spectrum would be obtained with a slit 1' in width.- This admits of a sharply defined spectrum.
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From page 118... ...
Only a continuous spectrum was recog nized; but the observer was confident that he would have seen dark lines in a solar spectrum of the same brightness. T E N N A N T ' S spectroscope is described as having a collimator of " short focus and considerable aperture." A s his image lens had a focus of 5 feet and an aperture of 4.G inches, the effective aperture of his spectroscope was probably very small.
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From page 119... ...
P E R R Y (p 423) leaves undescribed the essential constants of his instrument, but as the Casse grainian telescope which he used for an image leus had an angular aperture of only 1^° it is highly probable that the efitective apeiture was too small for the moderately wide slit.
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From page 120... ...
If we suppose the diameter of the pupil to be ^ of an inch, then all the light which reaches the retina comes from a portion of the image mirror 2 inches in diameter, and the remainder is useless. Again, since such prisms are usually equivalent to two or three pnsms of 6 0 ° , we see that the apparatus is inferior to a single-prism spectroscope of | of an inch effective aperture even for observing bright lines; and, in accordance with the principles explained in the theory of the instrument, it is greatly inferior to such a single-pnsm spectroscope for detecting dark lines.
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From page 121... ...
He saw the hydrogen spectrum with 1474 K bright on a continuous spectrum in the inner corona, and traced the last-named hue in the outer corona to a distance of more than 1° from the moon. He also detected dark lines in the spectrum of the outer corona, though they were seen with great diflficulty.
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From page 122... ...
Leaving out of account the integrating spectroscopes, because they do not distinguish between the lines due to the corona alone and those which have their origin in chromospheric light, we find that not a single observer, who has made use of an instrument which by description is as efficient as a single prism spectroscope of J-inch aperture, has failed to see the bright 1474 K line in the spectrum of the corona. A s to the height to which this line could be traced, at no eclipse when the effort has been made has this been less than 25' as a maximum except during that of 1878, when a single observer only (EASTMAN)
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From page 123... ...
IL POLARIZATION OF T H E CORONAL LIGHT, I t IS unnecessary to review the observations on this point with such minuteness as we have employed in discussion of the spectroscopic evidence. This is owing not only to the thorough manner in which Mr, R A N T A K D has done it, but also because there is no room for doubt, especially since Professor W R I G H T ' S admirable work at the eclipse of 1878, that the coronal light^s strongly polarized radially.
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From page 124... ...
e., suspended in it, falling into or projected from the sun, is a large quantity of solid or liquid material, which is at such a temperature as to be self-luminous. I t 18 this which yields the continuous spectrum free from dark lines.
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From page 125... ...
The material which produces the 1474 K line, and which may always be seen in the chromosphere spectrum, is, according to this criterion, as unmistakably denser than hydrogen as is magnesium vapor, or of iron vapor; but i f we accept the coronal spectrum as evidence of the existence of an atmosphere, we are, by exactly the same principle, driven to the conclusion that the 1474 K material is far less dense than hydrogen. The contradiction could not be more abrupt and inexplicable.
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From page 126... ...
This supply cannot come from parabolic comets unless those of small perihelion distances are more abundant than those of great. I am not aware that there is any evidence, founded either on theory or oljservation, which cannot be explained by the greater bnghtness of comets near the sun, and hence their greater liability to discovery, for such a distribution of perihebon distances.
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From page 127... ...
g., the fifth column contains the results of calculation under the assumption that the density decreases as the the sixth power of the distance from the sun's center I t will be seen that i t makes little difference what laws of distribution be assumed for the neighborhood of the limb; the polarization is there feeble, not far from 12 per cent., and increases continuously outwards. Since we have seen, however, that not more than'one-fourth or one-fifth of the light of the lower corona can be diffused sunlight, even eight times the polarization theoretically possible would not yield the percentage observed.
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From page 128... ...
that the coronal streamers aie formed by matter repelled from the sun 18 objectionable on account of what it implies. Sincp the nits are often many times darker than the streamers, i t follows that nearly all the white hght comes from such ejected matter.
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From page 129... ...
The relation of x' to the apparent angular distance from the moon's limb is, i f we represent this angle by a, given by the equation X ''=E whence we conclude that in order that any portion of the sun's surface may give a sensible amount of light to p, its angular distance from the moon's limb shall be not many tunes greater than ^ / f ^ ; in other words, the distance within the geometrical shadow of the moon, where light » 2E due to diffraction may be found, is limited to inches rather than miles. Thus i t would appear that the corona can have no explanation founded upon this phenomenon of light; and, indeed, there would be no escaping the conclusion i f the suppositions at the base of the argument are correct.
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From page 130... ...
First, the change can only be rapid during a very brief interval after second contact and before third contact, so that i t might readily escape observers whose attention is not specially directed to i t ; while, on the other hand, the camera cannot give evidence of rapid changes since, in the case of the outer corona, the ex posure cannot be very short. Secondly, there is another reason why such changes should not be very great, namely, the rapid diminution in brightness of the sun's disk in approaching the limb.
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From page 131... ...
Certainly, in view of the facts that it is only found in the closest proximity to the photosphere and that i t lies near the most briUiaiit portion of the spectrum where diffused light would do most to blot i t out, we must consider its visibility under any circumstances as a proof of extraordinary brightness. The diffraction theory also demands that the relative proportion of chromospheric light should dimmish in leaving the sun, hence the spectrum of the outer corona should more closely resemble that of ordinary feeble daylight.
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From page 132... ...
According to the diffraction theory the corona from such a sun must be strongly polarized at its base, where this light is relatively strong, and continuously decrease oirtwards; a couclnsion exactly contrary to the dicta of the old theory and in perfect accordance with observation. Thus, without contradicting any principle of physics, the difiiaction theory accounts for all the observed features of the corona with a single exception, namely, its more minute structure.
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