202 



NA TURE 



\yan. 14, 1875 



trum of the vapour is the simplest (single-lined), in the 

 core of the arc it is complex (many-lined). Now, in the 

 case of some of the elements present in the Sun, we have 

 a spectrum as complete as that we get in the core of the 

 arc, in others only a ine or two, so far as we know at 

 present. In fact, we have hydrogen and the metals of 

 the alkalis and alkahne earths and the metals of the iron 

 class with almost complete spectra on the one hand, and 

 on the other only a few lines indicating the presence of 

 such metals as zinc and lead. 



" Nor is this all. A most diligent search has been made 

 for metals of the tungsten, antimony, silver, and gold 

 classes among the metals, and entirely without success. 

 Dealing, however, with the metals the record of which is 

 most obvious in the solar spectrum, hydrogen, magne- 

 sium, calcium, sodium, and the metals of the iron group, 

 the order we have given is not only the order in which 

 they would be met by a body entering into the solar 

 atmosphere, but it is the order of the old atomic weights. 

 Further, although it is true that at present we do not 

 know much about the spectra of the stars, we do know 

 that the stars with the simplest spectra are stars which 

 only give clear indications of hydrogen, or hydrogen and 

 magnesium, or hydrogen, 3nagnesium, sodium, and so on. 

 A star as it gets gradually older may apparently give us a 

 spectrum belonging to a gradually increasing depth of 

 the solar atmosphere as it exists at this moment. 



" So far we have said nothing about metalloids ; that is, 

 those elementary bodies, such as oxygen, carbon, nitro- 

 gen, sulphur, and the like, which make up more than half 

 of the parts of our planet most easily got at. Of metal- 

 loids in the chromosphere none have been detected, but 

 a year ago a paper was presented to the Royal Society 

 pointing out that their record would appear not to be 

 entirely absent from the solar spectrum ; in fact, that we 

 have exactly such a record as we should expect if this 

 lai'ge class of bodies existed in a comparatively cool part 

 of the atmosphere at some height above the hotter lower 

 strata. It was also shown that, granting this, we could 

 explain the various classes of stars in the heavens by 

 supposing that as a star got older and colder the metal- 

 loids were enabled to exist lower down in the atmosphere, 

 and thus to change the character of the spectra of stars 

 bright and hot into that associated with those which are 

 dim and possibly colder, until at last the metalloidal rain, 

 so to speak, falling on the metals below, gives the mate- 

 rial of a future crust. It will be seen, then, that the work 

 since 1871 has been assuming more and more a chemical 

 character, and associated with this are physical questions 

 of the greatest interest, not only bearing on the kinetic 

 theory of gases, but which may eventually help us to 

 follow more intelligently than we can now the matter 

 of a nebula till it forms part of the cooled crust of a 

 planet. 



" The present line of inquiry, then, is to determine the 

 chemical nature of a section of the Sun's atmosphere 

 reaching from the photosphere to the extreme limit of the 

 corona, some hundreds of thousands of miles away. This 

 with the old conditions of observation, would have been a 

 hopeless task to accomplish. But, side by side with the 

 results to which we have drawn attention, new methods of 

 investigation have been introduced, and among these the 

 development of spectrum photography deserves first 

 mention. The spectrum of the corona can now be photo- 

 graphed with the same ease as the prominences were 

 photographed in 1S60, and if such photographs can be 

 obtained, it is certain that the work of four minutes will 

 in all probability be more valuable than laboratory work 

 extending over as many years. But even spectrum photo- 

 graphy would not have been applicable under the best 

 conditions unless side by side with it an instrument 

 had been introduced which is destined to effect 

 a great revolution in astronomical obsei-vation. In 

 the Siderostat we have an instrument, suggested by 



Hooke and perfected by Foucault, which enables us to do 

 away with telescope stands and their equatorial mountings 

 altogether. This is effected by moving a large, perfectly 

 plain mirror in front of the object glass of a telescope, 

 the telescope itself being horizontal and at rest. This 

 arrangement permits of spectroscopes and photographic 

 apparatus being attached to the eye-piece end of the 

 telescope of even greater dimensions than the telescope 

 itself. The special and novel method of attack to which 

 Mr. De la Rue referred as having been suggested to the 

 Council of the Royal Society can now be guessed by our 

 readers ; and unless we have missed the mark altogether, 

 it should now be seen that the work of the proposed 

 expedition of this year is the fruit and crown of the work 

 begun in i860 and carried on by the English and other 

 civilised Governments since that time " 



We have little to add to the foregoing, except that it 

 appears to us a sad thing, and little to the credit of the 

 leaders of astronomy in England, that such strong argu- 

 ments should have to be put forward at all in favour of 

 eclipse observations. Every total eclipse of the Sun 

 ought ta be observed as a piece of the national business 

 luith as great a regularity as the transit of the Moon over 

 the meridian of Greenwich. Nay, we may go further, 

 and say with greater regularity ; for we know something 

 about the motion of the iVIoon, and we can predict her 

 place with some accuracy, but he would be a bold man 

 who would predict the shape and condition of the Sun's 

 surroundings in the forthcoming eclipse. Practical men 

 might possibly urge the greater utility of one kind of 

 observation, but a man of science who does this is to our 

 mind not a true man of science at all. 



Mr. Hind has sent us the following most valuable in- 

 formation regarding the actual conditions of observation, 

 referring at somewhat greater length to Siam, whither 

 Enghsh astronomers are invited by the King of Siam. 



" Although the course of the central line in this eclipse 

 is mainly a sea-track, yet in its passage from the Nicobar 

 Islands, in the Bay of Bengal, to Siam, better opportuni- 

 ties for the observation of totality will be afforded than are 

 likely to be again offered before the close of the present 

 century. 



"Adopting the elements of the Nautical Almanac, in 

 which the place and hourly motions of the moon are 

 derived from Hansen's Tables, I find the following points 

 upon the central line : — 



" If we lay down these points on the Admiralty Charts of 

 the Bay of Bengal and Province of Tenasserim (British 

 Burmah), we find the central line passing a little north of 

 Kaikul, in the Island of Camorta, Nicobars, and on 

 making a direct calculation for Kaikul, totality is found to 

 commence at ih. 21m. 383. local mean time, and to 

 continue 4m. 27s., the sun being at an altitude of about 

 70°. I take the position of Kaikul, 5h. 13m. 31s. E. and 

 8° ii''S N. The central eclipse,' passing from the 

 Nicobars, traverses Bentinck Island, where the maximum 

 duration of totality is 4m. 17s., and runs between Mergui 

 and Tenasserim, rather nearer to the former place than 

 to the latter. By direct calculations I find — 



