THE
ERUPTION
OF
PELÉE
53
tance from the place of observation to the east, for the morning, or to t h e west, for the evening. This cannot occur frequently; therefore, contrary to w h a t certain authors think, the absence of this second segment does not prove the absence of atmospheric dust. " Moreover, other related phenomena, which I will briefly enumerate, go to prove, in their turn, t h a t this abnormal dust has never been absent during two years. (b)
SOLAR
AND
LUNAR
CORONAS—ANTHELIA.
"After examining carefully the notes t h a t accompany our observations with the dynamic actinometer and which give exactly the condition of the sky about the sun,* I find t h a t the solar corona was clearly perceived for the first time a t Bagnères and a t Pic du Midi, July 26, 1902, t h a t is, about two months and a half after the great eruptions a t Martinique. B u t it was only beginning with the m o n t h of December of the same year t h a t it was seen permanently around the sun, or around the moon during the night. I t is still visible whenever the sun shows itself ; however, it seemed less luminous in 1904 t h a n during 1903.† " T h i s corona is composed of a sort of circular white halo, immediately surrounding the sun, and whose exterior contour is slightly tinted with copperred or purplish pink. The coloration can be seen only b y hiding the sun behind an obstacle somewhat distant from the eye, such as a tree, the summit of a house, etc. " T h e mean diameter of the colored ring, as measured very frequently a t Bagnères or a t Pic du M i d i , ‡was about 48°, varying from 46° to 50°, at the end of 1903; the width of this ring was a t t h a t time about 20°; the outer diameter of the corona was about 70°. A t the present time the m e a n diameter appears to be from 40° to 44°; the measurement is difficult, however, especially a t the present time, because the colored ring merges insensibly into the white halo of the interior, and into the blue sky of the exterior. " At the close of 1903, we frequently saw in the luminous halo some slight striæ, analogous to the filaments of the cirrus, forming a sort of network and showing to the eye the appearance of a very fine dust, irregularly stratified and lighted; sometimes this same appearance of a dusty network was perceived also at the exterior of the corona, which appeared then much larger (its exterior diameter was, on certain days, 130°, while under ordinary conditions it was about 70°). Finally, I saw this dusty network over almost the whole e x t e n t of the sky. These phenomena were not produced in 1904. * " A t Bagnères a n d a t Pic d u Midi, we t a k e observations e v e r y t h r e e hours, daily, w i t h a s t a t i c a c t i n o m e t e r a n d besides, when t h e s t a t e of t h e sky will p e r m i t , with a d y n a m i c actin o m e t e r (of a s y s t e m i n t e r m e d i a t e between those of Violle a n d of C r o v a ) . " †" T h e n a m e Bishop's R i n g is often given t o this c o r o n a . " ‡" F o r these m e a s u r e m e n t s a n d for all those t h a t m a y h a v e to b e m a d e on t h e p h e n o m e n a of atmospheric optics, I h a v e devised a special g r a p h o m e t e r , v e r y easily used, very convenient, a n d v e r y easy t o c o n s t r u c t . "