ASTRONOMY. 401 



necessary to be able to discriminate between degrees of illumination 

 which do not differ from each other by more than one-tenth the intrinsic 

 brilliancy of the full moon. He considers that the eye is more able to 

 detect small differences of light than a photograph is, and states that 

 the moon cannot be photographed in full daylight, even though it may 

 be easily seen. His investigations also lead him to think that even in 

 the clearest weather the atmos])heric illumination is three hundred times 

 as bright as it should be for it to be possible to obtain any image of the 

 corona. To these points Dr. Huggins has replied in the Observatory 

 for November. Dr. Huggins states that he has had no difficulty at all 

 in photographing the moon in full sunshine, and that the observatioUvS 

 of Professor Langley and others, of Mercury and Venus, which have 

 been seen as black disks before they reach the sun, proves that the 

 corona must have a sensible brightness as compared with the atmos- 

 pheric illumination. 



He also points out that Mr. Pickering fails to obtain any trace on his 

 l)hotographs even of the detects of his own instrument. Dr. Huggins 

 declines further discussion, preferring to wait the result of the work now 

 being carried on by Mr. Ray Woods at the Cape Observatory. Mr. Pick- 

 ering replies in Science for October 23, admitting the possibility of pho- 

 tographing the moon in full sunshine, but contending that these very 

 photographs of the moon supply an additional proof of his opinion that 

 the light of the atmosphere near the sun is more than three hundred 

 times too intense for it to be possible to ob tain a photograi)h of the 

 corona, since the skylight near the sun was lifty times as bright as 

 that near the moon, and coronal photographs to be of any use should 

 be able to record differences of illumination of only one-tenth the bright- 

 ness of the full moon. 



He explains the visibility of Venus and Mercury as being caused by 

 the refraction of the sun's light through their atmospheres, the black 

 disk being thus surrounded by a narrow luminous ring. [Nature.) 



RemarTcahle solar prominences. * — On the 26th of June, 18S5, a remark- 

 able prominence was noticed upon the eastern limb of the sun by M. 

 Trouvelot, of theMeudon Observatory. Its measured height was about 

 10'-5, or about a third of the sun's diameter, but it probably extended 

 somewhat further. At a point on the sun's limb diametrically oppo- 

 site, another immense prominence was seen nearly equal in height 

 to the first. A somewhat similar phenomenon — two opposite promi- 

 nences — had been observed on November 22, 1884. 



On August 16, 1885, at 9^ 25", M. Trouvelot observed a brilliant 

 prominence 4' in height, which by ll'» 20™ had increased to 9' 27". With 

 the increase in height, it diminished rapidly in brightness, and at 11'' 22'" 

 had completely disappeared. 



Peculiar prominences were also remarked by M. llicco, o-f the Palermo 

 Observatory, September 16-19, and by M. Trouvelot on September 27. 



* V Astronomic, 4 : 441-7, 

 H. Mis. 15 26 



