scope was no bad guide in that matter, and, thanks to his valuable researches, we are now able to photograph as well, if not better, at the extreme red end of the spectrum than we did at that time-years ago now-in the blue. Well, then, four years were consumed in the accumulation of these facts. I do not now intend to call attention to the whole of them, but I will take some instances, directing special attention to what happened with regard to the spectrum of iron. This is the final map produced up to a certain point. We have first the solar spectrum; below this are mapped all the lines of iron observed on one of the photographs which we obtained, including of course all impurities; and then follow the spectra of manganese, cobalt, nickel, chromium, uranium, cerium, and so on through the whole story. When that work had been completed in that manner we had to get rid of the impurities by the process which I have already explained, and at last we got what is called a purified spectrum, in which, along the horizon labelled iron we had only those lines left which we could not by any application of the principle which has been explained be shown to be due to the admixture of any other substance whatever. What then was the total result? The accompanying table (p. 320) will show the sort of corner in which we found ourselves after all this work had been accomplished. It gives the list of the iron lines which, after making every allowance for the existence of impurities, were found to coincide with lines in other substances. It will be seen, for instance, that the two short lines 390600 and 395423 coincided, the first with short lines in uranium, zirconium, and yttrium, the second with short lines in uranium, molybdenum, and tungsten. Similarly there are two shortline coincidences with zirconium, and no less than six with vanadium, and so on. The total gives the coincidence of the lines of all the elements under the conditions that I have drawn attention to. So that the sum total of this really very laborious inquiry with regard to iron was that in the region between 39 and 40, the region including H and K on that map, where, before the introduction of photography, scarcely any iron lines had been seen, and where only five solar lines I think had been mapped, photography gave us a total of nearly 300 lines in the solar spectrum, and it gave us sixty-two lines of iron. Of those sixty-two lines of iron only eighteen went straight; by which I mean that the remainder had short-line coincidences with the lines of other substances. So that the idea first thrown out by Kirchhoff, Ångström, and Thalen of the possibility of the coincidence of lines among the metallic elements was enormously intensified. It will be seen that the thing is absolutely reversed in the case of iron, and it might be the case also in other substances. The fact of a line not being coincident with a line in another substance was the exception, and not the rule. The ratio in the case of iron being as 44 to 18. It is amusing in the light of recent criticisms to go back to the old observations and to see with what pertinacity for the first two years we stuck to the possibility that the solar line or the iron line we were dealing with was a double line, and then, after we had to give that idea up, as the coincidences became of three, four, five, and sixfold complexity, we came to the conclusion that we were dealing with a common impurity. That of course was a point we could not settle until we had gone through all the chemical elements which were known to us, and it was going through so many substances which took up so much time. But there was another question which became striking, in this excessively minute anatomy of even a very small portion of the solar spectrum, for I should say that the small range of the spectrum represented here forms a portion of a map which, when completed, will be the sixteenth of a mile long, so that after all we were dealing with an excessively small portion of the total work which had to be done. Having there mapped that small region, where without photography it would have been difficult to see any lines at all, we got in almost twenty cases from one end to the other, instances in which there was absolutely no relationship at all between the brightness of the iron line on our photographs and the darkness of the corresponding solar line. These were carefully noted as "anomalous reversals," a term we coined in the laboratory at the time, and which we still use, although the word anomalous always suggests a very large amount of ignorance. In more ways than one, then, this work landed us in rather worse confusion than we were in before. What we had to face was I This map is too large and detailed to reproduce here. seem time to consider what the effect would be, supposing that a dissociation was really going on under our eyes without our knowing or imagining anything about it. Why, it may be said, did you pitch on dissociation? For the reason that the startling results really after all contained nothing that was new-nothing that was novel about them the least in the world, if we regarded them with an absolutely unbiased and receptive mind. Dissociation would undoubtedly account for all the variations of intensity observed on passing from one temperature to another, as already exemplified in the case of the calcium lines, and moreover the short common lines, should they turn out to be truly common, which we were getting in the case of all substances, might be simply the equivalents of those short common lines of calcium which for years past we had watched coming out of the salts of calcium when decomposition was taking place. No new theory was necessary. The appeal to the law of continuity, as I said before, was really open to us, and it seemed to be our duty to appeal to it, and it was also easy to see, before really one has inquired into the matter, that if nature had built up the inorganic world in the way we now know she has built up the organic world, that precisely these facts and none other would be those she would present to us. the only one visible in B, as the spectrum of the assumed elementary body a would be the only one visible in A. compound substance y, and the same considerations will hold "A lower temperature furnace C will provide us with a more good. The figures between the hypothetical spectra point to the gradual change in the interstices of the lines as the spectrum is observed near the temperature of each of the furnaces. will be seen in furnace C, and the strong lines of y will be seen in furnace D, all as thin lines. Thus, although in C we have no line which is not represented in D, the intensities of the lines in C and D are entirely changed. "Now if into the furnace A we throw some of this doubly compounded body y, we shall get at first an integration of the three spectra to which I have drawn attention; the lines of will first be thickest, then those of 8; finally a will exist alone, and the spectrum will be reduced to one of the utmost simplicity. "The same reasoning therefore which shows how variation in "This is not the only conclusion to be drawn from these con- intensity can most naturally explain the short line coincidences siderations. Although we have by hypothesis B, Y, and 8 all-lines which I have termed basic, for the line of a strong in A higher, that is, more compound forms of a, and although the is basic in B, C, and D, the lines of B strong in B are basic in strong lines in the diagram may represent the true spectra of C and D, and so on. these substances in the furnaces B, C, and D, respectively, yet, in consequence of incomplete dissociation, the strong lines of 8 "I have prepared another diagram which represents the facts on the supposition that the furnace A, instead of having a tempera ture sufficient to dissociate B, y, and 8 into a, is far below that stage, although higher than B. "It will be seen from this diagram (Fig. 33) that then the only difference in the spectra of the bodies existing in the four furnaces would consist in the relative thicknesses of the lines. The spectrum of the substances as they exist in A would contain as many lines as would the spectrum of the substances as they exist in D; each line would in turn be basic in the whole series of furnaces instead of in one or two only." We are therefore completely justified in asking whether these are not the differences in intensities of lines to which Kirchhoff and Ångström have referred, and it is quite easy to see that if we change the temperature of the furnaces in such a manner as to produce the strongest lines, owing to the greatest quantity of the vapour given off at any temperature, that the long lines produced at these different temperatures would vary, and the longest line produced in furnace D would not be the same therefore as the longest line produced in furnace A, so that in that way we can imagine a transcendental temperature giving a very long line to a particular substance, and that substance may exist highly compounded in another substance, and yet at a lower temperature it may only appear as an exceedingly short feeble FIG. 35.-Portion of a large map showing the lines most affected in 100 sun-spots observed at South Kensington. line. The result of this reasoning was, in short, to explain at once variations of intensity of the short feeble lines which were common to so many of the so-called elementary bodies. I am particularly anxious to point out that there is absolutely nothing new in these views. We have simply taken as our exemplar the behaviour of a known compound body, and then pushed the reasoning three or four stages further. We have gone just the safest' possible way, by the easiest possible stages, from the known to the unknown. I have now to refer, one by one, to the various tests which have been applied to these considerations, and I should now like to bring the first considerable test under notice. I shall show on a subsequent occasion the various laboratory methods that we possess of determining whether short lines are really the product of high temperature. I shall at once draw your attention to the fact that the short lines may be due, not merely to the work of high temperature, being thus truly produced by the tem Derature which we are employing, but they may be also the may indications of excessively complex groupings which are just dying at the temperature we are using at the time. So that if it be permitted to coin terms I should like to call some of the short lines hot-short lines, and others cold-short lines. We shall see the reason by and by. Now if this order of things is in any way as I have stated it, the first test that we have to employ is one of excessive simplicity. The differences between terrestrial and solar spectra indicate that if the view be correct differences should be seen in the spectra of the same substances observed in different parts of the sun. We should now have a very distinct notion of the enormous difference of temperature between the highest and lowest reaches of the solar atmosphere. The lowest region of the solar atmosphere that we can get at must be far hotter than the highest part we can get at, at all events in times of eclipses; the lines that we should see therefore in the hottest region of the sun should bring us very near to the effects of this transcendental temperature to which I have referred, and the spectrum of iron seen in this way should bring us in presence of the result of the highest temperature. Let us take then the flames as giving us the spectrum of the hottest part of the sun. Where are we to find the record of the coolest part? Now to get to this point we have had naturally to dismiss all the observations which have been made of the lines visible in solar prominences, of the lines thickened in solar spots and the like, because we know that in these prominences and spots we really are dealing with phenomena local to particular and highly heated regions. Dealing with the whole solar spectrum we know that we are dealing with the whole of the solar atmosphere, however great, however high that atmosphere must be. Therefore we know that the solar atmospheric spectrum, the Fraunhofer spectrum, cannot by any possibility give us what is going on in any particular region-it must naturally be the summation of what is going on in every region where any absorption of any kind 324 Lines affected neither in spots nor storms.. = hottest. How have these views been tested. The first attempt made to get light out of this inquiry was one which simply dealt with a long catalogue of lines observed by Prof. Young in the memorable expedition of his to Mount Sherman, where, at the height of between 8000 and 9000 feet, with perfect weather and admirable instrumental appliances, about a month was employed in getting such a catalogue of lines as had never been got before. But it was found that, although the result of this inquiry was absolutely in harmony with these views, still after all one wanted more facts. Therefore we have endeavoured to get some of the facts here. And the way in which they have been collected is as follows:-During the last two years the spectra of 100 sunspots have been observed in the observatory here-observed in a new fashion, and for a good reason I think. In this changeable climate it does not do to do as we began by doing to attempt The exces. to observe all the lines acted upon in a solar spot. sive complication, and the intense variation of a spot-spectrum from the ordinary solar spectrum, cannot be better shown than by throwing on the screen the spectrum of one of the sun-spots lately observed at Greenwich. The figure (Fig. 34) shows a limitel part of the solar spectrum, and the lines thickened in the spot-spectrum. It will be seen therefore that to tabulate the existence and thickness and intensities of these lines over the whole of the solar spectrum would be a work which it would be difficult to accomplish in a single day, even if the day were absolutely fine. So that was given up in favour of a limited inquiry over a small part of the solar spectrum; limited further by this, that we only get the twelve lines most affected in each spot on each day. In this way we insure a considerable number of absolutely comparable observations, and we can more easily compare the spot results with those which had been obtained in the observation of the brightest lines in prominences, because when we begin to observe lines in the solar prominences one naturally begins by observing the brightest lines first. So that by observing the darkest lines first in the case of spots, one has a fairer comparison. A diagram (Fig. 35) will show the result of our observations of 100 spots over a very limited part of the solar spectrum. We will begin by the individual observations. We have at the top the iron lines recorded among the Fraunhofer lines; below we have the iron lines recorded as iron lines by Ångström, who used an electric arc. Lower down we have the iron lines recorded by Thalen, who used the electric spark. It will be seen that there is a very considerable difference in the spectrum of iron as viewed by means of the spark and by means of the arc, and that there is an equal difference between the spectrum of iron in the sun, that is to say, in the whole sun, determined by the Fraunhofer lines, and the spectrum of either the arc or the spark. It is also to be noted that the solar spectrum is more like the spectrum of the arc than the spectrum of the spark. Since the relative intensities in all these cases are represented by the length of the lines, we have here an opportunity of observing and discussing the accuracy of Kirchhoff's statement that the iron lines in the sun correspond absolutely in intensity with the lines of iron seen in a light source here. It is necessary first of all to see which light source he fixes on, whether the arc or the spark. When this has been done it is found that the statement is really true with regard to neither. That however is a digression; to proceed with the diagram, descending from this general spectrum of iron which we get by the absorption of the whole atmosphere of the sun independently of the hottest region and the coldest region-descending from the general to the particular-and taking that particular part of the solar atmosphere where the spots produce their phenomena, let us see what are the results in the case of the spots? We have in the vertical lines a record of the lines which are affected in each spot, and each of the spaces included between the horizontal lines represents a particular spot, the date being given on the right hand side; and these 100 lines which we have here The diagram represent the phenomena produced by 100 spots. is a small portion of the larger map. Now the wonderful thing Now the importance of these statements depends on other statements which we can bring to confront with them. The next diagram shows the observations of 100 prominences observed between the years 1872 and 1876. (The diagram was thrown on the screen.) Prominences exist in a region of the solar atmosphere not very far from that occupied by the spots, but we have already seen that whereas the spots are produced by a downrush of cool material, prominences are produced by an uprush of hot material. Let us see therefore if any change is produced in the phenomena; whether we shall have exactly the same lines from the flames, or the prominences, as we have from the spots; whether we shall get the same information or no. Here are the facts with respect to Tacchini's observations :We begin as before with the whole absorption of the sun, Angström's map, and Thalèn's map. I think you will see a very considerable change; the iron lines (for we are only dealing with iron) most prominent in the prominences are vastly different from the iron lines most thickened in the spots. The difference is shown in the annexed diagram (Fig. 36), which represents those individual observations both of spots and flames treated in a certain way with reference to the discussion. I will at once We have, as before, explain to you what that certain way is. the three data to begin with, and we have treated the sun-spot observations so that the lengths of the lines will represent the number of times they have been seen in 100 sun-spots; the line at wave-length 4919 5, for instance, has been seen seventy-two times; that line, in fact, has been seen more than any other; the one at 50050 some forty times, and so on; very many lines having been seen less than ten times. In another part of the same diagram we have summarised the individual results obtained from Tacchini's observation of prominences in exactly the same way. The line 50175 was seen in 66 prominences out of 100. But why I am particularly anxious to show this diagram is this, that it brings out the perfectly natural factfor it is the natural fact-that over this region of the spectrum, at all events, no iron lines affected in the spots are visible in the prominences. If we assume that the region occupied by prominences is hotter than the region occupied by spots, that higher region ought to do this work, and it ought to be a work of simplification. Therefore I say it is a perfectly natural result, and not one to be wondered at, that in the spectra of the flames there is no line coincident with any of the lines seen frequently widened in the spots. Now we have these three solar spectra here which we can compare one with the other. First of all we have the iron spectrum of the sun taken as a whole. Then we have next the spectrum of spots, which we know to be hotter than the sun taken as a whole. Then we have the spectrum of flames, which we know to be hotter than the spots. It will be seen that the story, as it runs from the top of the diagram downwards, is a story of greater simplicity, as it ought to be, and it was explained in the diagram which I exhibited before I began to show these results of absolute hard facts. It will be seen that the sim plicity brought about by the reduction of lines actually seen as to number, is accompanied by the appearance of new lines (produced by the transcendental temperatures) in these regions. This first discussion of a large number of spectra and of spots, as compared with storms, is, I submit, in absolute harmony with the view of the dissociation of the elementary bodies by the solar temperature suggested by Sir Benjamin Brodie in 1867, and therefore I may further add that to me, at all events, it is absolutely inexplicable on any other view. THIS J. NORMAN LOCKYER (To be continued.) INTERNATIONAL MEDICAL CONGRESS HIS Congress, which opened by an informal reception at the College of Physicians on Tuesday, has so far been a real success. It has brought together something |