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Now, supposing such a portion of the solar photosphere is observed as is shown in the space 8 s', fig. 34 (s being the solar disc), the images of this portion will give us the spectrum R V, showing the dark lines due to the absence of certain images as explained above; and no other portion of the disc produces any effect whatIt is very essential to remember this. We are in fact analysing under such circumstances the part ss of the disc, and no other part.

ever.

If a spot or a facula be crossed by s s', then the spectrum we get is no longer that of a uniformly, or

FIG. 35.

S

almost uniformly, bright part of the solar disc. If s s' (fig. 35) represent an enlarged view of the spot and the space included by the slit, then this last, seen separately, will be as s s'; and the spectrum will consist of a number of images of s s' ranged side by side, so as to - form a strip, as R V in fig. 34. Hence at the top and bottom of this compound spectrum there will be two narrow solar spectra corresponding to the parts s P and s' P'; next to these will be two narrow spectra of the penumbral parts P U and P' U'; and about the middle a narrow spectrum corresponding to the umbral

part u u', all these spectra forming one compound spectrum, whose red end is towards the left (assuming the dispersion to be as in the case illustrated in fig. 34) and its violet end towards the right. The nature of the penumbral and umbral spectra will be stated further on. It is by comparing these spectra with the adjacent solar spectra that the spectroscopist is enabled to form an opinion as to the nature of the spots, and to make inferences as to the general physical constitution of the Sun.

Similar remarks apply to the case where a portion of facula, or pores, or mottlings, or any other features of the solar disc, fall within the space s s'. All such peculiarities tend to produce peculiarities in the resulting compound spectrum; since the image of the portion s s' is repeated along the whole length of the spectrum R V after the fashion already described.

But, having considered these comparatively simple cases, let us deal with the subject which has of late attracted so much attention-the visibility of the spectrum of the prominences when the Sun is not eclipsed. Further on the exact nature of the prominence spectrum will be considered; but in this place I note only respecting it that it consists of bright lines.

Now, suppose that P P' is a prominence, s s' the edge of the Sun, and ss' the space included by the slit. Then p' s', as in the former cases, produces a solar spectrum (which, however, commonly presents certain peculiarities when belonging to the edge of the Sun's disc); the part p p' includes a portion of the

prominence, and gives a prominence-spectrum which we may suppose to be represented by the bright lines at C and F and near D. But it will also give a solar spectrum, for the light of our own illuminated air comes from the space included within the slit ss; and as our air is illuminated by solar light, it produces (according to rule 4, page 128) a solar spectrum. Also the part sp will give a solar spectrum due to the illuminated air. Now, the prominence P P' is absolutely obliterated from view by the illuminated air, which extends all round (and over, be it remembered) the place of the Sun. Since, then, if we looked at the space s s' alone,

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with whatever telescopic power, and with whatever contrivances for reducing the glare of light, the portion p p' of the prominence P P' would be wholly invisible to us, why, it may be asked, should the lines c, F, and the one near D-which are in truth but coloured images of the part p p'-be visible, although the spectrum of the illuminated air falling within s p' is spread over these lines precisely as the illuminated air is itself spread over the prominence? The answer is easy. The whole of the light of the illuminated air within the small space s p' is spread over the large space shaded with cross lines in fig. 36, and is reduced in intrinsic brightness in corresponding proportion.

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On the other hand, the light of the prominence-matter within p p' is spread only over the three lines shown in the figure (and a few fainter ones), and is therefore proportionately but very little reduced. Hence if we only have enough dispersive power, we can make sure of rendering the prominence-lines visible, for we get the same luminosity for them whatever the length of the spectrum, the only effect of an increase of length being to throw the bright lines farther apart; whereas the atmospheric spectrum which forms the background

FIG. 37.

will obviously be so much the fainter as we spread its light over a longer range.

By this plan we get a certain number of images of a portion of a prominence-a mere strip so to speak; and we can get any number of such portions, and in any direction as compared with the Sun's limb. For example, if s s' (fig. 37) be the Sun's limb, P P' a prominence, we can get from such a strip as s s' the spectrum R V. And obviously since the length of the bright lines tell us the length of the part p p' in figs. 36 and 37, we can, by combining a number of such parallel

strips as s s', learn what is the true shape of the prominence P P'.

But the plan can be applied to show the whole of a prominence. For let us suppose that in place of a narrow strip as s s', in figs. 36 and 37, we have a space such as is shown in fig. 38, through which, but for the intense brightness of the illuminated air, the prominence P P' would be visible. Then the part s s' of the Sun will produce a solar spectrum-altogether impure, of course, on account of the great width of s s', and brighter than the solar spectrum produced by s'p' in the case illustrated by fig. 36 in precisely the proportion that s s' in fig. 38 is greater than s'p' in fig. 36.

FIG. 38.

SPS

Red

Orange

Blue-green

All the remainder of the space, including the prominence P P', will give an impure solar spectrum due to the illuminated air, and very much brighter than in the cases illustrated in figs. 36 and 37, because so much more of this light is admitted through the open slit. Three coloured images will be formed of the prominences (other fainter ones need not be considered), one red at C, one orange near D, the other greenishblue near F. These images will be as bright (neglecting variations in the intrinsic brilliancy of the prominence) as the corresponding lines in the cases illustrated by figs. 36 and 37; but they will of course not

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