OF ARTS AND SCIENCES : MAY 10, 1870. 219 



it is bright enough for the colors to be distinguished, the red is outside 

 and the blue inside. 4. This arrangement of the colors, as well as the 

 dimension of the circle, indicate that a corona is not produced by refrac- 

 tion or reflection in crystals of ice, but by interference. The following 

 experiments which I shall now exhibit to the members of the Academy, 

 will illustrate this subject. When light is sent through the intervals be- 

 tween straight and parallel lines, which have been nicely ruled upon 

 glass, a series of colored fringes, parallel to the lines, results from the 

 interference between rays which pass through different openings. If 

 the glass were ruled with concentric circular lines, close together, these 

 colored bands would become circular, and surround the source of light. 

 By a rapid rotation of the ruled lines in theirown plane, subjective rings 

 result from the parallel fringes. In order to produce the required rota- 

 tion without a material axis, which would intercept the rays of light from 

 the eye of the observer, a platform is turned rapidly by clock-work. The 

 border of this platform is covered with cloth. The circular frame in 

 which the graduated glass is set rests upon this cloth, with its plane at 

 right angles to the platform, and is rotated by friction. Friction-rollers 

 at the sides and top hold it in its place, in the absence of any material 

 axis of rotation. If concentric black circles are accurately drawn upon 

 paper, and then photographed upon glass, on a greatly reduced scale, 

 the photographed plate might be substituted for that on which circular 

 lines had been scratched. Again, if a plate of glass is covered with 

 india-ink, and then concentric circles are scratched upon the black sur- 

 face, leaving the intermediate black rings, the same optical experiment 

 can be performed. All three of these methods have been tried, but the 

 finest and neatest circles were obtained by the last method ; and the ex- 

 perimental result is very beautiful, especially if the ruled glass is placed 

 immediately in front of the object-glass of an opera-glass. 



Although artificial coronae of great beauty can be produced in these 

 ways, it is obvious that the coronae of nature must have a much simpler 

 origin. And theory shows, that if lycopodium powder, the particles of 

 which are small and spherical, and of uniform size, is sprinkled upon 

 glass, a luminous spot, seen through the glass, will be surrounded with 

 several coronae, which, if less bright than those produced by the con- 

 centric rulings, on that very account have a greater resemblance to those 

 known in Meteorology. It appears that, in this indiscriminate sprink- 

 ling, myriads of minute openings are left everywhere on the plate, 

 enough being found in the required places for producing the colored 



