512 RECORDS 



of air from a tank of compressed air. The sound produced in 

 this way was inaudible on account of its high pitch, the wave 

 length being only about three-eighths of an inch. The whistle 

 was placed at one of the conjugate foci of a parabolic metallic 

 mirror, a sensitive flame being placed at the other conjugate 

 focus. A transmission grating made of wood and resembling 

 somewhat a portion of a picket fence was then interposed in the 

 path of the reflected sound waves, and it was found that when 

 the sensitive flame was shifted to one side, as many as four posi- 

 tions of maximum effect were obtained on each side of the cen- 

 tral beam of sound. With this apparatus, the wave length of 

 sound, when the waves were short like those used, could be 

 measured within one per cent. 



Mr. Levison's method consisted in the use of a specially con- 

 structed color screen most dense at the centre and fading off to 

 clear glass at the edges, which was placed close to the photo- 

 graphic plate. The size and density of the screen was adjusted 

 as nearly as possible so that the image of the inner corona made 

 by a suitable lens fell on the part of the plate covered by the 

 screen while the image of the outer corona passed through the 

 clear glass. The color screen was made from a lens of colored 

 glass with sharp edges, which was cemented into a recess in a 

 plate of clear glass ground to receive it. Two screens were 

 made, one of orange-yellow glass and one of dark greenish- 

 blue glass. In testing these screens at the time of the eclipse, 

 an arrangement of telephoto-lenses was used, but unfortunately 

 the exposure was not long enough to give any image at all of 

 the outer corona through the clear glass, although a consider- 

 able impression of the inner corona was produced through the 

 orange-yellow glass, but none through the bluish-green glass. 

 This should give some idea of the relative actinometric intensity 

 of the light from the inner and from the outer corona. 



Mr. Levison also presented a short note on the action of 

 Canada balsam on photographic plates. In making the experi- 

 ments with color screens he noticed that Canada balsam that 

 had been baked hard, when placed in contact with a sen- 

 sitive plate, or separated from it by a layer of carefully selected 



