304 PROCEEDINGS OF THE AMERICAN ACADEMY 



" The characteristics of the corona are as decided as those 

 of the halo. The arrangement of the colors, in which the red 

 occupies the outside and the violet the inside, points to inter- 

 ference, and not to refraction, as the physical cause of these 

 colors. It is an exhibition of diffraction on a large scale, 

 similar to the experiment of looking at a small flame through 

 a piece of thin glass, which has been sprinkled over with 

 lycopodium powder, or which is covered with minute particles 

 of smoke or moisture. In all these cases we see the flame 

 surrounded by colored rings, which are larger as the particles 

 of powder, which pave the way for the interference of the 

 light which passes through their interstices, are smaller. 

 The corona is produced by the cumulus cloud, in which 

 the water exists in a vesicular state ; not unlike in structure 

 to a piece of network, which is known to occasion inter- 

 ferences in light similar to those which are here attributed to 

 the cloud. Coronae are much smaller than halos. They are 

 between 3° and 12° in diameter. From the size of the corona 

 in any case, it is possible to calculate the size of the vesicles 

 in the cloud. According to the indications above mentioned, 

 the appearance of the 3d of December is to be classed among 

 coronae, and not among halos. Mr. Levering had proposed to 

 illustrate the subject by scattering lycopodium powder on 

 glass, and looking through it at a flame. But the number and 

 the size of the gas-burners with which the room where the 

 Academy assembled was lighted were not propitious for such 

 an experiment, which requires a single and small source of 

 light." 



Mr. G. P. Bond gave the results of observations recently made 

 at the observatory of Harvard College, upon two of the inner 

 satellites of Saturn, Tethys and Enceladus. " The perma- 

 nence of the mean motions of the latter over a period of several 

 thousands of its years was mentioned as an interesting fact. 

 Its mean distance also agreed nearly with that derived from 

 the periods and distances of the outer satellites. Should this 

 be sustained by further observations upon the two nearest 



