12 



KNOWLEDGE 



[Janiary 1, 1895. 



29m., and that of Fig. 4, taken at 2h. 22m. on the same 

 day. 



It is curious to reflect tliat but for what we must call the 

 happy accident of the nice adjustment of the apparent 

 diameters of the sun and moon, it is highly probable that 

 these spectographic photographs of the sun, taken by 

 M. Deslandres and Mr. Hale, would have been the first 

 intimation we should have had of the existence of chromo- 

 sphere and isromineuces. The mean apparent diameter of 

 the moon is distinctly smaller than that of the sun, being 

 under SI i minutes of arc as against a little over 32 minutes. 

 Were it not, then, for the eccentricity of the orbits of 

 earth and moon, we should never have a total eclipse at all. 

 Even as it is, so nicely adjusted are the apparent diameters 

 of the two bodies that it is not very uncommon for a 

 central eclipse to be both annular and total ; annular for 

 those countries where the eclipse takes place near sunrise 

 or sunset, total for those where it takes place near noon. 

 That is to say, the slight excess in the distance from the 

 moon of i^laces near the circumference of the hemisphere 

 in daylight, over places near the centre of that hemisphere, 

 suffices to reduce the apparent size of the moon, from being 

 a very little greater than that of the sun to being a very 

 little less. 



Were the orbits of the earth and moon circular, or were 

 the moon of equal density to the earth, its mass and 

 distance remaining unchanged, we should never have an 

 eclipse. Its period and its tide-producing power would, 

 under the latter hypothesis, remain the same. Bat its 

 volume would be changed, its diameter being diminished 

 by one-seventh. The greatest diameter it could ever 

 possess, even when in the zenith and at perigee, would be 

 only 2!»:f minutes, 2^ miautes of arc smaller than that 

 of the sun, even at apogee. A total eclipse would then 

 be an impossibility, for even under the most favourable 

 circumstances possible only six- sevenths of the sun could 

 be concealed. 



Had this been so, the sun's corona would have been 

 utterly unknown to us, and no one could have suspected 

 the existence around the sun of so wonderful and bi'.iinr 

 an appendage. There is nothing in the appearance of the 

 sun itself to suggest it. The disc shows a perfectly well- 

 defined and regular outline, unbroken by any excrescences, 

 except on the rare occasions when a facula makes a 

 scarcely perceptible projection. And even if we had 

 suspected the existence of such a body, we should have 

 found it hopeless to search for it. 



Nor should we have been much better off with regard to 

 the prominences. These were first seen by Staunyan 

 diiring the eclipse of 170G. Fraunhofer applied the 

 spectroscope to astronomy in 1814 ; Kirchhoff interpreted 

 the Fraunhofer lines in 1859. But though the existence 

 of the prominences was known, though the spectroscope 

 was in use, it was not until 1S08 that they were actually 

 seen in daylight. Indeed, Huggins had definitely described 

 the method by which they were to be rendered visible 

 more than two years before it was successfullv put in 

 practice. How long, then, would it have been before the 

 method of observing chromosphere and prominences would 

 have been stumbled upon if no one had known of their 

 existence '? 



Probably the history of our knowledge of these appen- 

 dages of the sun would have been somewhat as follows : 

 Lockyer, Secchi, Young, and others would have drawn 

 attention to the occasional reversal of the lines of 

 hydrogen over sunspots. It is unlikely that by chance an 

 observer would bring into his spectroscope both a briUiant 

 prominence and one of the hydrogen lines at the same 

 lime ; but with the increased application of photography 



to astronomy, the photographic spectra of sunspots would 

 begin to be studied. Then the observation of Young, 

 made some years ago, of the reversal of the H and K lines 

 over a spot and in its neighbourhood, would have been 

 confirmed, and later would have been extended to faculie 

 generally. This would have brought us down to the 

 commencement of the work of Hale and Deslandres, who 

 might not improbably have followed the same lines that 

 they have done under existing circumstances ; and by the 

 use of the double-slit spectroscope, and the invention of the 

 spectro-heliograph, they would have demonstrated to us, 

 for the first time, the existence of the prominences. 



It is scarcely possible to exaggerate the interest which 

 such a discovery would have excited ; yet the advance 

 which would have been involved by the discovery of the 

 prominences — previously undreamed of on the supposition 

 we have made — would have been scarcely greater than 

 that which the use of the doulile-slit spectroscope promises 

 to us. We are able, by its means, to separate, as it were, 

 one particular shell of the sun from all others ; or rather to 

 isolate one single gas in the solar atmosphere and to study 

 it alone. M. Deslandres has photographed the sun by 

 one of the bright spaces between two of the Fraunhofer 

 lines that is to say, by purely photospheric light. The 

 resulting picture corresponds, as we should expect, to that 

 shown to our eye by the telescope, or on an ordinary 

 photographic plate, but with the differences in brightness 

 between the spots and faculie and the general disc accen- 

 tuated. Again, he has photographed the sun by the light 

 of one or other of the Fraunhofer lines themselves ; for 

 these are of course relatively, not absolutely, dark ; that is 

 to say, he has photographed the reversing layer itself, by one 

 or other of the gases contained in it, just as it the sun and its 

 photosphere were removed. Finally, he has photographed 

 the sun, as in the present photographs, by the light of the 

 bright K lines, by glowing calcium vapour— that is, situated 

 at the base of the chromosphere, but above the " reversing 

 layer." It still remains to photograph the sun by the 

 third and narrowest line of the triplex K line, and so 

 obtain a record of a higher stratum yet of the solar atmos- 

 phere. The new solar records, therefore, offer us the 

 prospect of a far more intimate acquaintance with solar 

 structure than we could have ventured to hope for a few 

 years ago. 



PHOTOGRAPHS OF THE SOLAR 

 CHROMOSPHERE. 



By H. Deslandres. 



[M. Deslandres, l)y whose courtesy we are enabled to 

 submit to the readers of Knowleixie the four accompany- 

 ing photographs of the salar chromosphere, taken by him 

 at the Paris Observatory, on April 10th and 11th, 1894, 

 furnishes the following description of the instrument with 

 which they were taken.] 



The originals of the photographs were taken with the 

 following apparatus ; — 



1. The Foucault siderostat, constructed in 1869. 



2. An object glass of five inches aperture and three 

 metres focal length. 



3. A spectro-heliograph, or registering spectrograph 

 with two slits, adjusted to isolate the bright ray of 

 calcium. 



This spectrograph consists of a collimator fifty centi- 

 metres in length, a flint glass prism of sixty degrees, and 

 an object-glass of one metre focal length, so that the image 

 on the plate is magnified, and is exactly five centimetres 

 in diameter. 



