356 SPECIAL RESULTS IN THE TJRANOLOGICAL 



December, 1601 ; aud in more recent times, on the 10th of 

 June, 1816, in London, when it could not be discerned 

 even with telescopes. A peculiar, not sufficiently explained, 

 state of the several strata of the atmosphere in regard to 

 transparency must be the cause of this equally rare and 

 curious phenomenon. Hevelius remarks expressly, that, in 

 a total eclipse on the 25th of April, 1642, the sky was 

 covered with sparkling stars, the air being perfectly clear, 

 and yet, with the very various magnifying powers which he 

 employed, the Moon's disk continued without a trace of 

 visibility. In other also very rare cases, only some por- 

 tions of the Moon are faintly visible. In ordinary cases the 

 disk appears, during a total eclipse, of a reddish hue, the 

 colour being, indeed, of the most various degrees of inten- 

 sity, passing even, when the Moon is far removed from the 

 Earth, into a fiery glowing red. Whilst, more than half a 

 century ago (29th of March, 1801), I was lying at anchor 

 off the Island of Baru, not far from Cartagena de Indias, 

 and observing a total lunar eclipse, I was exceedingly struck 

 by seeing how much brighter the reddened disk of the 

 Moon appears in the sky of the tropics than in my northern 

 native land ( 571 ). The whole phenomenon is known to be 

 the result of the refraction of the rays, since, as Kepler very 

 correctly expresses it (Paralip. Astron. pars optica, p. 893), 

 the Sun's rays are inflected in their passage through the 

 Earth's atmosphere ( 572 ), and thrown into the cone of shadow. 

 The disk, whether it be of paler or darker red, is never uni- 

 form in colour throughout. Some parts always show them- 

 selves of deeper tint than others, and the change of colour 

 takes place progressively. The Greeks had a peculiar theory 



