300 



NATURE 



\yuLy 28, 1881 



source. We have the upper and lower pole, the globule of iron 

 volatilising, and the vapour, both in the arc, properly so-called, 

 and the accompanying flame. It is obvious that if we throw the 

 image of the arc on the slit we can examine the vapour without 

 getting any light from the pole. It is also obvious that if we 

 arrange the slit horizontally while the current is passing in a 

 ver ical direction from one pole to the other, we shall be able. 



Fig. 23. — Chromosphere with jets (Tacchini). 



by moving the slit upwards, to see if there are any differences 

 observable in the vapour, first in the region where we have 

 intense boiling and volatili ation going on, and in the neces- 

 sarily cooler region where the arc is in contact with the outer air. 

 Photographs taken in this manner show what is really observed 

 in the case of iron under these circumstances. Whether we use the 

 artifice of a horizontal arc with a vertical slit, or a vertical arc 

 with a horizontal slit, does not matter, provided we keep the slit 

 immersed in the light of the arc, and thus reflect the light from 

 the poles, and at the same time arrange the slit so that we 

 can compare the light in the interior portion of the light source 

 with the light nearer its buundaries — if we take all these pre- 

 cautions ne shall then get in the case of every substance such a 

 result as here exhibited (Fig. 25). We have in the centre a complete 

 spectrum, its intensity being gradually toned down, and some pf 

 the lines being left behind as we look up and down towards the 

 boundary w here we have the spectrum of that portion of the arc 

 which was the last to retain its luminosity in consequence of its 

 cooling. If we take horizons from the central portion of the 

 diagram to the point furthest distant from that central axis, 

 we find at last the light becomes absolutely monochromatic. 

 The iron vapour at this distance from the central axis really was 

 only radiating to us the vibration rendered visible to us by that 

 one line. As we get nearer and nearer the centre of agitation 

 the spectrum becomes more complex, until at length when very 

 near the central axis we get a great many short lines introduced, 



^^'T^mBsam 



Fig. 24.— Arrangement for obtaining long and short lines, 

 horizontal arc on slit plate of .spectroscope. 



Image of the 



so that the specti-um at that 1 oint is most complex. This I am 

 anxious to draw attention to with some insistence, because we 

 shall understand at once the terms long and short lines from this 

 diagram, and abuut those long and short lines there will le a 

 great deal in the sequel. 



The figure shows the much more simple spectrum of sodium. 



In all ca^es under the conditions mentioned it is quite 



easy to obtain photographs of longs and shorts ; the longest line 

 in the middle is D, that to the left the line is the green, and w e 

 find that one line excels all the others, and reaches a greater 

 distance from the central axis of the photograph. 



An electric lamp can be arranged to show the long and short 

 lines of sodium on a screen ; the arrangement is rather a deli 

 calc one, but the point is that we have not, as in the case of the 

 other electric lamps, vertical poles, but horizontal one% and we 

 have a vertical slit close to the horizontal poles in the very middle 

 of the lamp, so that if the experiment is carried far enough we 

 can then pruve the accuracy of the statement that the line is an 

 image of the slit, because the slit generally melts, and we see 

 the shape of the lines varying on the screen as the melting goes 

 on. The lines are of different lengths : the yellow is longer 

 than the green, the green longer tlian the red, and so on. 



Results obtained by this method have a very important bearing 

 upon every question connected with solar spectroscopy. When 

 these spectra were observed — the spectra of the longs and shorts, 

 of course we had a perfectly new set of phenomena to deal » ith. 

 In all preceding spectra all the lines hid been practically shown 

 of the same length, or else the lengths had represented their 

 intensities. But here we had, in the case of each chemical sub- 

 stance, to deal with the remarkable fact that when that chemical 

 substance was examined in this way, some of the lines were long 

 and some of them were short, and the question naturally arose, 



Fig. 25. — Spectrum of sodium, showing the long and short lines. 



how is it that some of the lines are long and some of themshurt ? 

 That question was an exceedingly difficult one to answer : I 

 do not know that it has been thoroughly answered yet ; but while 

 researches were being made for the answer to this question cer- 

 tain general statements became possible which are of very con- 

 siderable importance to us in our inquiry. Such a general 

 statement as this, for instance, that if we tale, say, some iron, 

 observe its spectrum, and then mix some manganese with it, and 

 observe the spectrum of the mixture ; if the quantity of man- 

 ganese is very small, we shall only get the longest line of man- 

 ganese ; if the quantity of manganese is increased, the next 

 longest line will come in ; and so on. So that if the spectrum 

 of any specimen of iron was photographed, it was at once easy 

 to see whether there was an impurity of manganese in that iron. 

 If you make the admission that the spectra of iron and man- 

 ganese, and so on, were the spectra of bodies not decomposable 

 at the temperature which yiu were employing — if, for instance, 

 there was a great quantity of manganese existing as impurity in 

 the iron — you got a great many lines, and of coiu'se with the 

 quantity of admixture the number of lines would go on increas- 

 ing until you had 50 per cent, of each, when you would have 

 the greatest number of lines of iron and the greatest number of 

 lines of manganese you could ever get together, but in no case 

 then w ould you get all the lines of iron, or all the lines of man- 

 ganese. 



The great importance of this result was, that it enabled any 

 spectroscopist, or any chemist w ho chose to take the trouble 

 and devote the time to it, to examine as to the existence of im- 

 purities in different substances ; not to determine the absolute 

 amount of impurity, but enabling him to say that in specimen A 



