314 



NATURE 



\Aiig. 15, 1872 



namely, 4 in. in diameter. As it would be inconvenient to 

 mount such an instrument equatorially, it is proposed to fix it in 

 the meridian in a horizontal position, and reflect the sun in the 

 direction of its axisby meansof a flat mirrormoved by a heliostit. 

 There cannot be any doubt about the fact that the image so pro. 

 duced would be nearly free from optical distortion, if the inter- 

 posed mirror did not in'roduc± a new source of error. The 

 ditficultv of producing a plane mirror is well known ; and there 

 is a dilTicuIty in maintaining its true figure in all positions ; there 

 is also a liability of the disturbance of the rays by currents of 

 heated air beHveen the mirror and object-glass : moreover, with 

 such an instrument, position-wires cjuld not be defined with 

 sharpness on the photographs. On the whole, greater reliance 

 may be placed on a method which admits the e-iiistence of a dis- 

 torting influence, but has at the same time means of checking 

 and controlling it numerically. 



Great attention has been paid by me at various times to tho>e 

 effects of distortion which might arise from the process of drying. 

 The results to which the experiments lead seem to prove tluit 

 there is no appreciable contraction except in thickness, and that 

 the collodion film does not become distorted, provided the rims 

 of the glass plates have been well ground : this point is a funda- 

 mental one. But in such observations as that of the transit of 

 Venus, no refinement of correction ought to be neglected ; hence 

 fresh experiments will be undertaken to set at rest the question 

 whether distortion of the film really takes place when proper 

 precautions are taken. This will be done both by the method I 

 have employed before, and also in accordance with M. Pasclien's 

 proposal to measure images of such reticules as above de- 

 scribed : this reticule might, as he has suggested, be photo- 

 graphed during the transit of Venus, so that each plate would 

 thus bear data for the correction due to unequal shrinking, if such 

 were to take place. 



It has been olijected by some astronomers who have casually 

 examined solar photograms that the limb of the sun appears as a 

 consequence of the gradual shading off, even under a small 

 magnifying power, not bounded by a sharp contour ; but the 

 measurements of such photograms which have been made during 

 the last ten years of pictures, taken under the most varying con- 

 ditions which influence definition, have proved that even the 

 worst picture leads to a very satisfactory determination of the 

 sun's semi-diameter and centre ; moreover, an independent exami- 

 nation of this question by M. Paschen gave as the result that the 

 mean error of a determination is only i o'ooS millimetre with 

 a sun-picture of 4 Paris inches in diameter ; this corresponds to 

 ± o"'l35, and it is nearly three times less than that resulting 

 from a measurement with the Koningsberg heliometer. 



Nevertheless it will be seen from the foregoing remarks that I 

 have not hesitated to arouse your attention to the fact that As- 

 tronomical Photography is about to be put to the severest test pos- 

 sible in dealing with such a fundamental problem of astronomy 

 as the determination of the sun's distance from the earth. An 

 intimate knowledge of the subject, however, and experience 

 with respect to work already accomplished in the Kew ten-year 

 solar observations, inspire me with a confident anticipation that 

 it will prove fully equal to the occasion. 



So much for performances to be looked forward to in the 

 future : now let me briefly review what Astronomical Photo- 

 graphy has already undoubtedly accomplished. 



In the first instance the possib lity proved of giving to the 

 photographic method of observation a trustworthiness which 

 direct observations can never quite obtain, wdi render the re- 

 sults of our discussion of the ten years' solar observations at 

 Kew more fiee from doubts than those observational series on 

 the Sun's spots which have preceded ours. The evidence of a 

 probable connection between planetary positions and solar acti- 

 vity, and the evidence which we have published on the nature of 

 spots as depressions of solar matter, could never have l^cen 

 bi ought forward but for the preservation of true records of tlie 

 phenomena through a number of years, while the closer agree- 

 ment of the calculated results in reference to solar elements is 

 itself evidence of the intrinsic truthfulness of the method, and 

 gives the highest promise that our final deductions, which will 

 be completed in the course of the ensuing year, will not be un- 

 worthy the exertions which I, in conjunction with my friends \j. 

 btrwait and B. Loewy, have constantly devoted to this work 

 dirir.g a period of fully ten years. Not only will some doubt- 

 ful qutsiions be set finally at rest by it, but new facts of the 

 greatest intertst will result, bearing on the laws which appear to 

 govern solar activity. 



By mthing, however, would the claims of photographic 

 observation, as one of the most important instruments of scien- 

 tific research, seem to be so thoroughly well established as by 

 the history of recent solar eclipses. It will be recollected that 

 in 1S60 for the first time thi solar origin of the prominences 

 was placed beyond doubt solely by photography, which pre- 

 served a faithful record of the moon's motion in relation to these 

 protuberances. The photographs of Tennant at Guntour, and 

 of Vogel at Aden, 1S6S, and also those of the American astro- 

 nomers at Burlington and Ottumwa, Iowa, in 1869, under 

 Profs. Morton and Mayer, have fully confirmed those results. 

 In a similar manner the great problem of the solar origin of 

 that portion of the corona which extends more than a million 

 of miles beyond the body of the sun has been, by the photo- 

 graphic observations of Col. Tennant and Lord Lindsay in 1S71, 

 set finally at rest, after having been the subject of a great 

 amount of discussion for some years. 



The spectroscopic discovery in 1S69 of the now famous green 

 line, 147.^ K, demonstrated undoubtedly the self-lumino-sity, 

 and hence the solar origin of part of the corona. Those who 

 denied the possibility of any extensive atmosphere above the 

 chromosphere received the observation with great suspicion ; 

 but in 1870 and again in 1S71 it was fully verified. So far, 

 therefore, the testimony of spectroscopic observations was in 

 favour of the solar oiiginof the inner corona. 



Indeed the observations of 1 87 1 have proved hydrogen to be 

 also an essential constitutent of the "coronal atmosphere," as 

 Janssen proposes to call it — hydrogen at a lower temperature 

 and density, of course, than in the chromosphere. Janssen was 

 further so fortunate as to catch glimpses of some of the dark 

 lines of the solar spectrum in the coronal light, an observation 

 which goes far to show that in the upper atmosphere of the 

 sun there are also solid or liquid particles, like smoke or cloud, 

 which reflect the sunlight from below. Many problems, how- 

 ever, even with reference to the admittedly solar part of the 

 corona, are unsettled. The first relates to the nature of the 

 substance which produces the line 1474 K. Since it coincides 

 with a line in the spectrum of iron, it is by many considered 

 due to that metal ; but then we must suppose either that iron 

 vapour is less dense than hydrogen gas, or that it is subject to 

 some peculiar solar repulsion which maintains it at its elevation, 

 or other hypotheses may be suggested for explaining the fact. 

 Since the line is one of the least conspicuous in the spectrum 

 of iron and the shortest, and as none of the others are found 

 associated with it in the coronal spectrum, it seems natural, as 

 many have done, to assume at once that it is due to some new 

 kind of matter. But the observations of Angstrom, Roscoe, 

 and Clifton, and recently those of Schuster regarding the spec- 

 trum of nitrogen, render it probable that elementary bodies hava 

 only one spectrum, and since in all experimental spectra we 

 necessarily operate only on a small thickness of a substance, we 

 cannot say what new lines may be given out in cases where 

 there is an immense thickness of vapour ; and hence we canni, 

 conclude with cerlainty that because there is an unknown lint 

 in the chromosphere or corona, it im]>lies a new substance 

 Another problem, the most perplexing of all, is the reconcilia- 

 tion of the strangely discordant observations upon the polarisa- 

 tion of the coronal light ; but I will at once proceed li. 

 the points on which photography alone can give us decisivi. 

 information. 



The nature and conditions of the outer corona (the assemblago. 

 of dark rifts and bright rays which overlies and surrounds tb*; 

 inner corona) was .very incompletely studied ; and the questio ■ 

 whether it is solar was not finally settled in the opinions oi' 

 astronomers of high repute. Some believed it to be caused by 

 some action of our atmosphere ; and others supposed it due to 

 cosmical dust between us and the moon. The bright light of 

 the corona and the prominences most undoubtedly cause a cer- 

 tain amount of atmospheric glare ; and although it is difficult 

 to see how this is to account for the rays and rifts, it would be 

 rash to deny that it may do so in some m mner yet to be dis- 

 covered. It is quite certain that some of the phenomena 

 observed just at the beginning and end of totality are really 

 caused by it. A light haze ol meteoric dust between us and 

 the moon might give results much resembling those observed ; 

 but when we come to detads this theory seems to be doubtful. 



Heie photography steps in to pave the way out of the existing 

 doubts. If the rays and rifts were really atmospheric, it would 

 hardly be possible that they should present the same apptaranc* 

 at different st..tions along the line of totality ; indeed, they would 



