excited by 5790.
other higher orders.

Similar complications, of course, occur at

If we could excite the iodine absorption lines one at a time there would be no difficulty in finding out how the groups are built up, but this is impossible with present facilities.

By varying the voltage at which the mercury lamp operates, and by filtering the light through bromine vapour, some clues have been obtained regarding the relations existing between the absorption lines and the lines forming the groups, but a complete analysis has not yet been made.

In fig. 4 I have given a diagram of the groups up to the

Fig. 4.

seventeenth order excited by the green mercury line of the quartz lamp operating at 115 volts. The doublets (lines 6 and 7) excited by the Cooper-Hewitt lamp appear in all of these groups, though they are relatively faint owing to the reversal of the exciting line, and these doublets are brought into coincidence in the diagram.

When the iodine is excited by the lamp operating at 60 volts, lines 2, 4, 5, 6, and 7 appear in the group of

O order, line 6 being of course the unresolved complex of emission lines corresponding to the absorption lines covered by the green mercury line. Line 7 is the companion line which, together with the "R.R." line corresponding to absorption line 3, forms the doublet of 0 order. doublets of higher order lie immediately below, the increasing distance between the components being very apparent.

The

Now line 2 is a companion line to the R.R. line corresponding to absorption line 4, indicated also by line 6 in the diagram. These two lines form another doublet of zero order. The higher orders do not lie immediately below, but drift to the left, as indicated by the dotted lines. This is due to the fact that the constant in the second term of the formula is a little less than in the case of the first series of doublets considered, in other words the doublets are closer together.

In the group of the first order the main line of this series. of doublets can be separated from the main line of the other series only in the fourth order spectrum of the grating. In the third order group it is so far detached, that it was confused for a long time with line 5 of the first order group. If we compare the orders 0 and 6 we shall see another case of this kind: If it were not for this diagram arrangement of the groups, we should probably assume that the first line to the left of group 6 corresponded to line 1 in group 0, whereas the diagram shows clearly that it corresponds to line 2. Moreover, it appears in the 60-volt excitation, which does not bring out lines 1 and 3.

In the construction of the diagram it is, of course, necessary to leave blank spaces for the missing orders, otherwise the corresponding lines will not lie on a smooth

curve.

It is a little difficult to explain in words just how this diagram is to be interpreted, though it is clear enough if the theory of the group formation which I have given is understood. All of the lines with the exception of 6 in the 0 order group must be companion lines, line 6 being made up of the unresolved R.R. lines. In the case of the doublets, the superposition of which forms the other groups, we must distinguish between what I have called the main line and the companion. As we run up the diagram the main lines should lie on curves intersecting line 6, for example, the dotted curve shown which belongs to the 2, 6 doublet.

I have not yet been able to identify certainly any other main lines, though I suspect that the one corresponding to companion line 9 descends from line 6 on a curve sloping

to the left at a lesser angle than the dotted curves, i. e., at about the angle taken by companion line 3.

Various modifications in the conditions of excitation have been made with a view of establishing which absorption lines are responsible for the various doublets.

For example, it was found that the lateral emission and the end-on emission of a Cooper-Hewitt lamp showed a very different intensity distribution in the green mercury line, as shown by figs. r and s, Plate VIII., which were made with a very fine plane grating by Dr. Anderson. If the iodine vapour is excited by the lateral emission of the lamp, as with the "light-furnace" companion line No. 1 appears in addition to the strong doublets. See 0 and 1 orders of fig. j, Plate VII. After several failures I succeeded in obtaining a record of the iodine resonance excited by the end-on emission, and in this spectrum companion line No. 2 appeared also. Now companion line No. 1 does not appear in the case of excitation by the quartz arc operating at 35 volts, and the short wave-length satellite of the green line is weaker, with respect to the main line, in this case, than in the case of the Cooper-Hewitt lamp, as is shown by figs. t and u, Plate VIII. (t being the Cooper-Hewitt line and u the quartz arc). This makes it appear probable that companion line No. 1 arises. from the excitation of the absorption line which is in coincidence with the short wave-length satellite.

Companion line No. 2 is probably due to the excitation of absorption line No. 4. It comes out with excitation by the "end-on" emission of the Cooper-Hewitt lamp owing to the broadening of the main line which occurs under this condition, and for the same reason it is the first line to appear when the terminal voltage of the quartz arc is increased. No very definite conclusions have been drawn from the numerous experiments which have been made with the exciting light filtered through bromine vapour and nitrogen tetroxide. With a potential of 90 volts on the quartz arc companion lines 4 and 5 appear. If the exciting light is filtered through bromine vapour contained in an exhausted bulb about 30 cm. in diameter, line No. 5 disappears in the groups of order 0 and + 1. In the third order group line No. 5 is much stronger than 4 and bromine filtration of the exciting light equalizes the intensity. Line No. 4 must therefore be due to the excitation of an absorption line which is not in coincidence with a bromine line, and which is first covered by the mercury line when the lamp operates at 90 volts. This seems to be absorption line No. 5, while the other

component, which is removed by filtration of the exciting light through bromine, is probably due to absorption line 6.

With a potential of 110 volts on the lamp, companion line No. 3 appears, and this also is removed by the bromine filtration of the exciting light, as is shown by figs. p and q, Plate VIII., in which q is the resonance spectrum obtained when the exciting light is filtered through bromine. It appears to be due to the stimulation of absorption line 7 which is in coincidence with a bromine line.

The difficulty in interpreting the results obtained is due to the fact that the mercury line widens both to the right and left as the voltage increases, so that two absorption lines may be attacked simultaneously. If this happens, we can differentiate between them only if one of them is in coincidence with a bromine line and the other not. What is most needed just now is one or more other filters similar to bromine vapour, but I have not been able to find anything with sufficiently narrow lines, though I have tried a number of vapours which looked promising. What would be still better would be to alter the wave-length of a narrow exciting line so as to cause it to pass by degrees from one absorption line to the next.

Excitation by the Yellow Lines.

The resonance spectra excited by the two yellow lines have not been completely investigated as yet, though a large number of photographs have been made. Each yellow line excites a series of nearly equidistant groups which resemble roughly the groups excited by the green line. Six pairs of these groups, from 1 order to +4 order, photographed with rather low dispersion are shown by fig. i, Plate VI. In this case the excitation was by the quartz mercury arc operating at 140 volts, the green line having been cut off by means of a glass trough filled with a solution of eosine. Some difficulty was found in securing the spectrum excited by the Cooper-Hewitt arc, as the yellow lines are comparatively weak in this case, but satisfactory results were finally obtained with the light furnace, the iodine tube being wrapped around with a sheet of gelatine stained to a deep orange-yellow.

In this case each yellow line excited a series of doublets, but both series were much more irregular than the series excited by the green line.

The separation of the components of the doublets excited by the 5790.7 line varied in an irregular manner from 2.1 to 5-6 ÅU. In the case of the excitation by the 5769.6 line we have also a series of doublets, though the companion line is missing at the zero order, in other words the R.R. line has no companion. The separation of the components of the doublets is less irregular in this case, varying from 4.8 to 54 AU. The table of wave-lengths will be given in the ÅU. communication following this one.

XXVIII. The Series Law of Resonance Spectra. By Prof. R. W. WOOD, Johns Hopkins University, and Prof. M. Kimura, University of Kyoto*.

IN

N the previous communication a general account of the results which have been obtained, up to the present time, on the resonance spectra of iodine has been given.

The present paper will deal with the measurements of wave-length of the lines in the groups, and the subject of the series law which governs their spacing.

The wave-lengths in the lines in the groups of 0 and +1 order were determined from plates made in the fourth order spectrum of a large plane grating with a telescope of 3 metres focus. They are correct probably to 0.01 AU. The groups +2, +3, and +4 were made in the second order spectrum, and the higher order groups in the first order spectrum.

The series which has been most definitely determined, and to which the greatest amount of study has been given, is the series of strong doublets excited by the Cooper-Hewitt lamp.

The two components of each doublet appear to be of equal intensity, although, in the case of two or three, a different ratio appears in the photograph as a result of absorption. It was found, as has been stated in earlier papers, that the first order group, which is usually recorded with the component of shorter wave-length three or four times as intense as the other, comes out with its lines of nearly equal intensity if the lateral branch of the iodine tube is cooled to zero, while the right hand component disappears entirely if the light from the tube is passed through a large glass bulb containing iodine vapour before it enters the spectroscope.

In studying the series law it has been found necessary to

*Communicated by the Authors.