1870.] The approaching Total Solar Eclipse. 483 



studying the corona. The telescope should be accurately driven by 

 clockwork, and a dark iris-disc, if one may so describe an arrange- 

 ment which would be the converse of an iris-diaphragm, might be 

 employed with advantage to hide the light of the prominences and 

 chromosphere. If the field of view were several degrees in diameter, 

 and the dark disc at the beginning of totality concealed a circular 

 space extending a degree or so beyond the eclipsed sun, the observer 

 might first examine with great advantage the outer parts of the 

 corona, and gradually extend his scrutiny to the very neighbour- 

 hood of the prominences." 



A question of extreme importance, which seems fairly within 

 the range of the available modes of research, consists in the deter- 

 mination whether the outer and extremely faint parts of the corona 

 show any sign of prolongation towards those regions where, as we 

 know, the zodiacal fight extends. The whole of that portion of the 

 heavens along which (speaking with reference to the place of the 

 sun) the zodiacal light is ordinarily visible, is above the horizon 

 during most total eclipses. Further, the dark region corresponding 

 to the place occupied by the moon's shadow in our atmosphere, 

 extends at the beginning and end of totality over a very wide range 

 of sky, first on the western and then on the eastern side of the 

 lunar disc. Along this region the faint glow of the zodiacal fight 

 ought to be perceptible if sought for under favourable circumstances. 

 Among such circumstances are, of course, a clear sky and an elevated 

 station. But there is one condition which, so far as we know, has 

 never yet been attended to. The maximum darkness of a solar 

 eclipse comes on so rapidly that the eye is yet dazzled by the light 

 when totality is in progress. Nor does totality last long enough 

 for the eye to acquire the power of recognizing faint differences of 

 illumination. This fact serves to explain the failure of observers, 

 hitherto, in detecting the delicate phenomenon we are now consider- 

 ing. There appears to be good reason for believing that the search 

 would be conducted with a much better prospect of success if the 

 observer who undertook it were in the first place to keep his eyes 

 as much as possible in darkness until totality had fairly commenced ; 

 and in the second, to hide the whole of the corona from view while 

 searching for the zodiacal fight. This could be very easily managed 

 by arranging beforehand a black disc so as to conceal the sun at the 

 time of totality from an eye placed at a certain aperture, through 

 which the observer should conduct his search, during totality, for 

 the faint light along the ecliptic. This method seems so promising, 

 and the inquiry itself is so full of interest, that we cannot but hope 

 some observer will be willing to devote himself specially to this par- 

 ticular subject. 



But we may safely expect from those who have volunteered to 

 take part in an expedition which will probably be by no means a 



