17- 



NATURE 



\Dcc. 3[, 1874 



good conditions, coupled with the further information 

 which we pubhshed on Tuesday, detailing the care 

 taken at Jassy to in?ure the accuracy of the obser- 

 vation of external contact at egress by Doctors Weiss 

 and Oppolzer, furnishes a good opportunity of re- 

 ferring to the whole question of such contacts, and of 

 pointing out an almost general omission in the scheme of 

 observations. ... 



A fev; ceneral considerations will show how, in the 

 opinion of some competent judges at all events, there 

 is a remedy for such a state of uncertainty as we have 

 described in the case of external contacts. We have first 

 the essential consideration which underlies the various 

 methods of utilising a transit, that when Venus is as near 

 to us as she is on the occasion of a transit — Venus, of 

 course, is always nearest to us when she is between us 

 and the sun — unless she be exactly between us and the 

 sun, so that we can use the sun as a screen or back- 

 ground, and see Venus moving like a black spot upon it, 

 she will not be visible to us at all, as her bright side will 

 be turned away from us. To point this statement we 

 may remark that this is not the case with Mars, the path 

 of which planet lies outside ours. Mars, in fact, is 

 brightest and best visible when nearest to us, and his 

 distance has been measured, as astronomers have just 

 measured the distance of Venus, by using the longest 

 possible base line on the earth and determining the appa- 

 rent change of place of Mars among the stars as seen from 

 the opposite points, thus using the stars as a background. 

 The r: ccesses, it is true, are different in their details, but 

 the s. me in intention. The special observations of 

 ingress, egress, nearest approach to sun's centre, and the 

 like, in the case of Venus, arise out of the fact that the 

 only available screen is a limited one and of a certain 

 shape, and, it may be said, are so many contrivances 

 which enable us to use the centre of the sun's disc, as we 

 use a star in the observations of Mars. In either case, of 

 course, whether we delermine the distance between the 

 earth and Mars or the earth aiid \'enus, we determine 

 the distance of the sun and the dimensions of the whole 

 solar system. 



Now, within the last few years it has been established 

 that the sun, with its sensibly circular boundary which we 

 see every day — the screen which we use in the case of 

 transits of Venus — is by no means the whole of the sun ; 

 it is only the central brighter portion of it. An exterior 

 nebulous mass, leebly luminous compared with the central 

 one, lies outside it, and in consequence of its feeble light 

 it is quite invisible to us, except during total eclipses of 

 the sun, when the moon cuts off the brighter light of the 

 central portion, and allows us to see the exterior, irregu- 

 larly-bounded one, extending for hundreds of thousands 

 of miles away into space in all directions. 



Although, as we have said, this exterior portion cannot 

 be seen, except during eclipses, in consequence of the 

 strong illumination of our atmosphere near the sun's 

 place, the lower brighter parts of it can yet be rendered 

 visible without an eclipse by the use of a spectroscope, 

 and it is no exaggeration to say that by the aid of this 

 instrument a large part of the sun outside that part of 

 it ordinarily visible can be seen as sharply and as con- 

 veniently as any part of the sun's surface can be observed 

 by a telescope. 



The method by which this is accomplished will be 

 easily understood by anyone who will take the trouble to 

 look at the flame of a candle, the wick of which has been 

 almost covered with common salt, through one of those 

 " drops," triangular in section, which form part generally 

 of a common lustre or a chandelier. A small prism will, 

 of course, be better still. If the " drop " or prism be held 

 close to the eye and upright, some four or five yards from 

 the candle, at such an angle that the flame can be seen 

 through it, a peifect yellow image of the wick and flame 

 will be seen. Besides this image there will be a bhize of 



colour to the right and left of it, but the yellow image of 

 the flame will be brighter than the rest. 



Now, common salt is a compound of sodium with 

 chlorine, which compound is decomposed by heat ; and 

 it is the vapour of the metal sodium set free which gives 

 us. at the heat of the candle flame, light of one colour 

 only, which cannot be dispersed or split up by the prism. 

 The flame of the candle, on the other hand, gives out 

 white light, which, being composed of light of all colours, 

 is split up by the prism ; so, while the prism has no 

 action on the one, it has an enormous action on the other, 

 and as a result gives us a perfect image of the flame, 

 built up by the simple light of sodium vapour, brighter 

 than the spectrum of the flame itself in that region. 

 Further, the white li^ht of the candle gives us no clear 

 image, because in fact there are millions of images of 

 every tint superposed ; so that we get but a confused 

 rainbow effect, due to the white light. The exquisite 

 sodium image of the flame is due to the fact that there is 

 no overlapping ; and again, the reason that the addition of 

 the salt to the ilame, while it scarcely increases the light 

 of the candle, gives us a spectral yellow image brighter 

 than the background, is easily explained by the fact that 

 in this part of the spectrum, as the coloured band is 

 called, the sodium light is helping the yellow light of the 

 flame, which gets no such help in other parts of the 

 spectrum. 



Now, we know as a matter of fact that the exterior 

 regions of the sun give a spectrum similar in character to 

 that given by the sodium vapour in the candle flame, and 

 that the sun itself gives us a spectrum similar to that of 

 the ordinary flame of the candle, and that it is because 

 our air is illuminated by light of this kind stronger than 

 the light of the external part of it that it is invisible 

 to us. 



To see, then, the external regions of the sun to which 

 we have referred, the physicist looks at them through a 

 prism, as in the candle experiment ; in fact, he uses many 

 prisms to spread out to the utmost the sun-light reflected 

 to us by our intervening atmosphere, which , sunlight, as 

 we have seen, has a spectrum similar in its nature to the 

 spectrum of a candle flame. When he has done this he 

 sees the images of the strange forms in these external 

 regions, as the yellow image of the candle was seen, the 

 light producing which was concealed by the brighter light 

 of the flame till the prism was brought into play. Of 

 course, he knows now exactly in what part of the spectrum 

 the light which they give out is to be found. He knows 

 that all round the sun there is an atmosphere of vividly 

 bright hydrogen, the light of which is red ; he therefoie 

 looks in the red part of the spectrum, and the atmospheric 

 veil being withdrawn by the prism in the way we have 

 stated, he is enabled to trace by the red light given out 

 by the hydrogen exactly what the hydrogen is doing, and 

 where it exactly is. He knows that magnesium is some- 

 times ejected from the sun with terrific force into this sea 

 of hydrogen, and he knows that the light of magnesium 

 va])Our is green, so he examines the green part of the 

 spectrum and so observes the exact size and shape of 

 these volcanic bursts of magnesium vapour. 



We then come to the point of this long digi'ession. 

 When we bring the spectroscope into play the sun is made 

 larger ; outside the round disc there is discovered a con- 

 tinuous envelope extending to various heights, which we 

 can observe. Our screen, therefore, is increased, and 

 exterior contacts are exterior contacts no longer, if we can 

 manage to see Venus passing over the newly-discovered 

 region before she reaches the disc. 



How, then, can this be accomplished ? There are three 

 ways in which this can be accomplished. We have first 

 that ordinarily employed in observations of the chromo- 

 sphere —as the newly-discovered region which surrounds 

 the sun and can be spectroscopically observed without an 

 eclipse is called. We have next a method devised by 



