Dec. 31, 1874] 



NA TURE 



^11 



Father Secchi ; and still a third, independenily hit on by 

 several investigators. 



In the method ordinarily employed, in order to avoid 

 as much as possible the overlapping of images of sensible 

 breadth (which prevented the white light of the candle 

 flame from giving us even a distant approach to a pure 

 spectrum), the light is allowed to fall on the prism through 

 a vei-y fine sHt of a certain height. On this slit an imnge of 

 the sun is thrown by a tine telescope. If the whole length 

 of the slit is immersed, so to speak, in this image, we 

 shall see nothing but the spectrum of the part of the disc 

 which falls on the sht. If it is only half immersed in it, 

 we shall see less of the spectrum of the disc, but we shall 

 see also the spectrum of the chromosphere, as the oblite- 

 rating effect of the reflection of the sunlight by our air 

 has been destroyed by the prisms. 



This spectrum will consist of bright lines, and if %ye can 

 manage to place the slit on the precise spot occupied by 

 Venus the lines will be broken, as the chromosphere will 

 be eclipsed in this part by the planet ; and we can follow 

 the planet's motion until the break in the line travels 

 down to the spectrum of the sun ; this will mark the 

 instant of exterior contact at ingress. At egress the pro- 

 blem is simpler, as the actual place occupied by the planet 

 prior to external contact can be seen by an observer set 

 to watch the sun's image on the slit of the spectroscope. 



An obvious objection to this method, if a better one can 

 be found, lies in the fact that Venus has, as it were, to be 

 " fished for " prior to external contact at ingress, and that 

 the slightest error in following the planet's motion would 

 render the mode of observation useless. 



The next method is one devised by Father Secchi. 

 Using a spectroscope as before, instead of throwing a 

 simple image on the slit, by an object-glass merely, he 

 throws a spectrum of the sun on the slit by means of 

 prisms, placed either before the object-glass or between 

 It and the slit. He slates that by this method the solar 

 disc is seen with its spots and edge quite clearly defined, 

 and that the spectral lines of the chromosphere are also 

 seen. Further, the slit can be opened wider with advan- 

 tage than under the first method. It is clear, therefore, 

 that when Secchi's method is employed, if it does all that 

 he says it does, observations of exterior contact would be 

 easy. 



The third method is a photographic one, and if it 

 succeeds at all would do away with the main objection to 

 the first two. A reference to the candle experiment will 

 make it quite clear. If we imagine for a moment the 

 white light of the ordinary flame of the candle to be 

 abolished, it is clear that we should see nothing but the 

 pure yellow image due to the monochromatic vapour of 

 sodium. Similarly, if we imagine the light of the sun 

 aboHshed, w9 should see the whole ring of the chromo- 

 sphere if we looked at it through a simple prism, as a ring, 

 or as a series of rings, according to the kinds of light 

 given out by the vapour of which it is composed (the 

 rings taking the place of the lines when we use a slit). 

 In this way the chromosphere and the coronal atmo- 

 sphere which lies outside it were actually seen in their 

 true ring-like form by Prof. Respighi and Mr. Lockyer in 

 the Indian eclipse of 1S71, the light of the sun being 

 temporarily abolished by the interposition of the moon. 



In the third method, then, instead of a slit, a disc is 

 used. All the sun is thus hidden, with the exception of a 

 very small ring at the extreme edge, underlying the 

 chromosphere. It is certain that the whole ring of 

 chromosphere can thus be photograpiied every day the 

 sun shines, as it is now observed on every such day by 

 Mr. Seabroke at the Temple Observatory at Rugby 

 School ; and it is believed that the lower surface of the 

 chromosphere can be thus photographed as hard as the 

 outline of the sun itself, for there are many favouring 

 conditions which, however, it would take us too long to 

 enter upon in this place. 



It is Clear that by the application of this method there 

 is a possibility of obtaining a whole series of photographs 

 both before and after Venus is seen on the sun, and it is 

 also clear that the method can only be tested on the 

 occasion of a transit. 



We know that Lord Lindsay's expedition, which has 

 been organised with a completeness which puts our official 

 programme into the shade, is to test Secchi's method, 

 and that Dr. Janssen was to use some spectroscopic com- 

 bination. The Italian p.arties, as we have already men- 

 tioned, were to limit themselves to external contacts as 

 observed by the spectroscope, but their Government 

 subsidy came so late that it is certain they were not 

 equipped in the most complete manner, and it is probable 

 that their original programme has been considerably 

 curtailed. 



AUhough the spectroscope forms no part of the eq;uip- 

 ment of the English parties, as it certainly should have 

 done, seeing that they intended to observe contacts more 

 than anything else, we may still hope that some of the 

 methods will have been tested, and that the value of the 

 aid they bring to obser\rations of external contact may be 

 determined. 



NOTES 



The Belgian Academy of Sciences have conferred upon Prof. 

 Huxley, Sec. R. S.. the dignity of Foreign Associate. Such a 

 step on the part of so very Catholic a body may make amends 

 for the anathemas of the Irish prelates. 



We are glad to be able to contradict a statement which has 

 appeared in some of the papers that Prof. Bunsen was about to 

 leave Heidelberg. He has, we learn, no intention of doing eo. 

 The loss of Professors Kirclihoff and Ivonigsberger is one which 

 this University will f.el most severely, and we cannot help 

 wondering what the authorities at Cailsruhe were about to 

 render it possible for two such men to be tempted away. Prof. 

 Kirchhoff has declined the directorship of die Solar Observatory 

 at Potsdam, and goes to Berlin as free Academician and as 

 Professor m the University ; Prof Konigsberger has accepted 

 the post of Professor at the large Polytechnic School iu Dresden. 



The scientific results to be obtained from Arctic exploration 

 wUl be carefully attended to in making the arrangements for the 

 forthcoming Arctic expedition. Each officer will take up a 

 special branch of scientific investigation, and will devote himself, 

 during the interval between his appointment and the sailing of 

 the expedition, to acquiring such knowledge as will enable him 

 to exert his energies most usefully. There will also be a civihan 

 naturalist or geologist in each ship, who will be carefully selected 

 •with reference to special knowledge and other qualifications. It 

 is possible also that an Engineer ofticer may accompany the expe- 

 dition, with charge of magnetic and pendulum observations. 

 Some of the men forming the ships' companies will also be 

 selected for their special qualifications. Among these, a dog- 

 driver, named Karl Petersen, formerly cooper at tlie Danish 

 setdement of Upernavik, has already been entered. There will 

 also be three ice quarter-masters in each ship, chosen from 

 the crews of the whalers, and one of the first duties of Capt. 

 Markham on his arrival in England will be to pioceed to Dundee 

 for the purpose of selecting and entering these men. Capt. 

 Markham was telegraphed for to Lisbon on the 20th, and is 

 expected to arrive in London this week. 



Lieut. Bellot, brother of the unfortunate Bellot, the Arctic 

 explorer, to whom we alluded in a recent number, has obtained 

 leave from the French Government to volunteer for the English 

 Arctic Expedition. 



On- Dec. 11, at 4.45 a.m., a severe shock of earthquake was 

 felt by Gen. Wansouty and two friends, who intended to spend 



