146 cosmos. 



peculiar and not sufficiently investigated diaphanic condition 

 of individual strata of our atmosphere. Hevelius states dis- 

 tinctly that, during a total eclipse on the 25th of April, 1642, 

 the sky was covered with brilliant stars, the atmosphere per- 

 fectly clear, and yet, with the different magnifying powers 

 which he employed, not a vestige of the Moon could be seen. 

 In other cases, likewise very rare, only separate parts of the 

 Moon are feebly visible. During a total eclipse, the disk gen- 

 erally appears red ; and, indeed, in all degrees of intensity of 

 color, even passing, when the Moon is far distant from the 

 Earth, into a fiery and glowing red. While lying at anchor 

 off the island of Baru, not far from Carthagena de Indias, 

 half a century ago (29th of March, 1801), I observed a total 

 eclipse, and was extremely struck with the greater luminous 

 intensity of the Moon's disk under a tropical sky than in my 

 native north.* The whole phenomenon is known to be a 

 consequence of refraction, since, as Kepler very correctly ex- 

 presses himself (Paralip Astron. ]jars Optica, p. 893), the 

 Sun's rays are innectedf by their passage through the at- 



* " On comjoit que la vivacite de la lumiere rouge ue depend par 

 uniquement de l'etat de l'atmosphere, qui refracte, plus ou moins affai- 

 blis, les rayons solaires, en les enflechissant dans le cone d'ombre, mais 

 qu'elle est modifiee surtout par la transparence variable de la partie de 

 l'atmosphere a traverslaquelle nous apercevons la Lune eclipsee. Sous 

 les tropiques, une grande sei'enite du ciel, line dissemination uniforme 

 des vapeurs diminuent l'extinction de la lumiere que le disque lunaire 

 nous renvoie." — Humboldt, Voyage aux Regions Equinoxiales, torn, iii., 

 p. 544 ; and Recueil d'Observ. Astronomiques, vol. ii., p. 145. " It may 

 easily be understood that the intensity of the red light does not depend 

 solely upon the state of the atmosphere, which refracts more or less 

 feebly the solar rays by inflecting them into the shadow cone, but that 

 it is especially modified by the variable transparency of that part of 

 the atmosphere across which we perceive the eclipsed Moon. Under 

 the tropics a great serenity of sky, a uniform dissemination of vapors, 

 diminish the extinction of the light which the lunar disk sends toward 

 us." Arago observes : " Les rayons solaires arrivent a notre satellite 

 par l'eftet d'une refraction et a la suite d'une absorption dans les couches 

 les plus bases de l'atmosphere terrestre ; pourraient-ils avoir une autre 

 teinte que le rouge?" — Annuaire for 1842. p. 528. "The solar rays 

 reach our planet by the effect of a refraction, and subsequently to an 

 absorption (partial) in the lower strata of the Earth's atmosphere. How 

 can they have any other colors than red ?" 



t Babinet declares the reddening to be a consequence of diffraction, 

 in a memoir as to the different share of the white, blue, and red Lights 

 which are produced by the inflection. See his Reflections upon the 

 Total Eclipse of the Moon on the 19th of March, 1848, in Moigno'a Re- 

 pertoire d'Optique Moderne, 1850, torn, iv., p. 1C56. " La lumiere dif- 

 fractee qui penetre dans l'ombre de la Terre, predoraine toujours et 

 memo a ete seule sensible. Elle est d'autant plus rouge ou orangee 



