. The book of photography; practical, theoretical and applied. instrument, which is much employedin eclipses, is termed a coionagraph.(The corona is photographed along withthe chromosphere.) The chromosphereacts as a slit, and needs no , H/3, and Hyare represented by threerings, D.^ by another, and so on. Forcomparison, line spectra may be madefrom these negatives by using a small stripof the ring spectrum and a cylindricallens. Very often the object glass iscorrected for photographic rays, when thefocus must be found by photography. 604 THE BOOK OF PHOTOGRAPHY. So
. The book of photography; practical, theoretical and applied. instrument, which is much employedin eclipses, is termed a coionagraph.(The corona is photographed along withthe chromosphere.) The chromosphereacts as a slit, and needs no , H/3, and Hyare represented by threerings, D.^ by another, and so on. Forcomparison, line spectra may be madefrom these negatives by using a small stripof the ring spectrum and a cylindricallens. Very often the object glass iscorrected for photographic rays, when thefocus must be found by photography. 604 THE BOOK OF PHOTOGRAPHY. Solar Prominences. Only some of the lines of any knownelement are given by the magnesium lines out of seven areknown ; two sodium lines out of eight;two iron lines out of two thousand, reasons for this have yet to be fullyworked out, but it is probably a questionof temperature and pressure. Theprominences of the sun are masses ofglowing gas, which stretch out forthousands of miles from the chromo-sphere. The ring spectra photographs. Fig. 808.—The Coud^ Equ.^tokial Telescope. of the chromosphere give the promin-ences, and show that not only do differentprominences vary in chemical constitu-tion, but that the same prominence hasdifferent compositions at different levels,because the different rings of colour withtheir prominences are of different spectroscope with its slit is directedclose to the edge of the suns imagewhere the chromosphere is known to method suggested by Sir WilliamHuggins, and applied by Janssen andLockyer, is used to diffuse the continuousspectrum, and the length of the hydrogenK line is observed. The length of the slitimage will be proportionate to the lengthof the section across the prominence. By causing the collimator slit to move alongthe prominence, its hydrogen shape maybe sketched. This method is in principleidentical with that employed in takingmonochromatic images of the sun. Otherlines of the spectrum wil
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