366 



NA TURE 



\AM01tst 30, 1877 



not to act under the law, " a gas when ignited absorbs 

 rays of the same refrangibility as those it emits." But 

 in fact the substances hitherto investigated in the sun are 

 really metallic vapours, hydrogen probably coming under 

 that rule. The non-metals obviously may behave differ- 

 ently. It is easy to speculate on the causes of such 

 behaviour, and it may be suggested that the reason of the 

 non-appearance of a dark line may be that the intensity 

 of the light from a great thickness of ignited oxygen over- 

 powers the effect of the photosphere just as if a person 

 were to look at a candle flame through a yard thickness 

 of ignited sodium vapour, he would only see bright sodium 

 lines, and no dark absorption lines. Of course, such an 

 explanation would necessitate the hypothesis that ignited 

 gases such as oxygen give forth a relatively large propor- 

 tion of the solar light. In the outburst of T Coronn 

 Huggins showed that hydrogen could give bright lines 

 on a background of spectrum analogous to that of the 

 sun. 



However all that may be, I have no doubt of the exist- 

 ence of substances other than oxygen in the sun which 

 are only indicated by bright lines. Attention may be 

 called to the bright bands near G, from wave-lengths 

 4307 to 4337, which are only partly accounted for by 

 oxygen. Farther investigation in the direction 1 have 

 thus far pursued will lead to the discovery of other 

 elements in the sun, but it is not proper to conceal the 

 principle on which such researches are to be conducted 

 for the sake of personal advantage. It is also probable 

 that this research may furnish the key to the enigma of 

 the D3 or Helium line, and the 1474 K or Corona line. 

 The case of the Dj line strengthens the argument in 

 favour of the apparent exemption of certain substances 

 from the common law of the relation of emission and 

 absorption, for while there can be no doubt of the exist- 

 ence of an ignited gas in the chromosphere giving this 

 line, there is no corresponding dark line in the spectrum 

 of the solar disc. 



In thus extending the number of elements found in the 

 sun we also increase the field of inquiry as to the pheno- 

 mena of dissociation and recomposition. Oxygen, espe- 

 cially from its relation to the metals, may readily form 

 compounds in the upper regions of the solar atmosphere 

 which can give banded or channeled spectra. This 

 subject requires careful investigation. The diffused and 

 reflected light of the outer corona coukl be caused by 

 such bodies cooled below the self-luminous point. 



This research has proved to be more tedious and diffi- 

 cult than would be supposed because so many conditions 

 must conspire to produce a good photograph. There 

 must be a uniform prime moving engine of two-horse 

 power, a dynamo-electric machine thoroughly adjusted, a 

 large Ruhmkorff coil with its Foucault break in the best 

 order, a battery of Leyden jars carefully proportioned to 

 the Plijckcr's tube in use, a heliostat, which of course 

 involves clear sunshine, an optical train of slit, prisms, 

 lenses, and camera well focussed, and in addition to all 

 this a photographic laboratory in such complete condition 

 that wet sensitive plates can be prepared which will bear 

 an exposure of fifteen minutes and a prolonged develop- 

 ment. It has been difficult to keep the Pliicker's tubes in 

 order ; often before the first exposure of a tube was over 

 the tube was ruined by the strong Leyden sparks. More- 

 over, to procure tubes of known contents is troublesome. 

 For example, my hydrogen tubes gave a spectrum photo- 

 graph of fifteen lines of which only three belonged to 

 hydrogen. In order to be sure that none of these were 

 new hydrogen lines it was necessary to try tubes of various 

 makers, to prepare pure hydrogen and employ that, to 

 examine the spectrum of water, and finally to resort to 

 comparison with the sun. 



The object in view in 1873, at the commencement of 

 this research, was to secure the means of interpreting the 

 photographs of the spectra of stars and other heavenly 



bodies obtained with my 28-inch reflector. It soon 

 appeared that the spectra of nitrogen and other gases in 

 Pliicker's tubes could be photographed, and at first some 

 pictures of hydrogen, carbonic acid, and nitrogen were 



made, because these gases seemed to be of greatest astio- 

 nomical importance on account of their relation to stars, 

 nebulx", and comets. Before the subject of comparison 

 spectra of the sun was carefully examined there was some 

 confusion in the results, but by using hydrogen the source 

 of these errors was found out. 



But in attempting to make a prolonged research in this 

 direction, it soon appeared that it was essential to be able 

 to control the electrical current with precision both as to 

 quantity and intensity, and moreover to have currents 

 which, when once adjusted, would remain constant [for 

 hours together. These conditions are almost impossible 

 to attain with any form of battery, but on the contrary 

 are readily satisfied by dynamo-electric machines. Ac- 

 cordingly, I sought for a suitable dynamo-electric machine 

 and motor to drive it, and after many delays procured a 

 combination which is entirely satisfactory. I must heie 

 ackno\\lcdge my obligations for the successful issue of 



Fig. a. — Braytcn's Petroleum Motor. 



this search to Prof. George F. Barker, who was the first- 

 person in America to procure a Gramme machine. He was 

 also the first to use a Brayton engine to drive a Gramme. 

 The dj namo-electric machine selected is one of 



