342 ANNUAL OF SCIENTIFIC DISCOVERT. 



ZODIACAL LIGHT. 



Tlie followino: letter from Cliacornac was published in the 

 "KraiU'r," ISCo: — 



" Tlic observation of the zodiaeal ligiitat tlie epoeh of the win- 

 ter solstice, in latitude 45°, is not a very extraordinary fact; bnt 

 as I do not know any notice of it, it may Ite useful to mention how 

 it ap))ears under cin innstances wiien it can l)e well observed. On 

 tlie 2:k\ Dc'cemljer, 18G4, we noticed here, at G.4.'j v. u., that an 

 intensely liuninons portion of tlie zodiacal liijht plainly detached 

 itself from the iiottom of a ^rlooniy sky. This could be traced to 

 aliont tin- constellation Pisces. The sky was but inililVercntly clear. 

 At the time of inaUiiij>: this observation I .sou;;ht to determine the 

 quantity of li<;ht which this portion emitted, compared with that 

 eniilled by the stars. For tliis jnirpose I dri-w on ])a|)er S(!vcral 

 bluek lines between while ones of the saini; breadth, all ccjually 

 distant. At seven in the evening, the zodiacal lifjlit ajipearin*^ in 

 all its intensity, I distinfrnislied, with dilliculty, the white lines, 

 which were about a millimetre distant from the others. This limit 

 of visibility was found even when precautions had been taken to 

 obtain a <jrreat sensiiiility of the retina. On tlu! ni;^lit of the 29th 

 and 3(tth December a diffused luminosit}', which ajjpeared to me 

 to paint the sky, was eertaiidy more intens(! than that seen on the 

 2."3il of the same month. Indeed, on the oOth, at 10 V. M., it was 

 possible to distinji^uish white lines separated only the third of a 

 millimetre from black lines of equal dimensions; it was impossible 

 to do so on December 2o. I next examined how this difi'used light 

 distrii)Uted itself, and saw that the most luminous j)art was above 

 a whitish i)liosphoreseent veil, which covered the horizon with a 

 light and luminous mist; the intensity of the luminosity was thcn-e- 

 fore not uniformly distributed. Employing a photometric appa- 

 ratus, and designating b\- unity the luminous intensit}- of the sky 

 in the neighborhood of the horizon, at 25° high we had 1.53 for 

 the expression of the intensity of the light of the skj' at this point; 

 overhead, the heavens deprived of stars, we had 1.15 for the in- 

 tensity of the light in the region of the zenith. Thus, ijroceediug 

 from the horizon, the luminous intensity of the sky increased up 

 to a height of al)out 25°, and then decreased as we rose from this 

 point to the zenith. At least, so it was on the 30th of December, 

 18G4. 



"If we admit that it maybe light from stars which thus illu- 

 minates our atmosphere during serene nights, we do not explain 

 why the stars should not be visible on a dark sky through a trans- 

 parent atmosphere. The fine divisions of the instrument used, 

 whilst they become visible when the atmosphere is less transpa- 

 rent, enable us to see a luminous veil intei-posing itself between 

 the light of the stars and the observer. We know that all light- 

 absorbing media radiate light ; this medium, then, absorbing the 

 light of the stars, radiates some light towards the earth. On the 

 30th December, from 10 to 11 P. M., and from midnight to 1 A. M., 

 the ))hosplioresceut veil was so intense that the milky-way could 

 hardly be seen. 



