82 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



whereas the color from Q' appeared contiguously on the left, the apparatus 

 being used like a half -shade. But the attempt was not of practical value for 

 incidental reasons. 



FIG. 44. 



FIG. 45. 



48. Experiments with a grating. Since in the observation of coronas the 



diffractions toward the right of a group of particles on the left are coordi- 

 nated with the diffractions toward the left of a group of particles on the right, 

 it seemed interesting to endeavor to reproduce the coronal phenomenon by 

 two identical and coplanar gratings with their rulings in parallel and at a 

 sufficient distance apart. The very large dispersion of the usual commercial 

 grating would here be an annoyance; but the circular grating constructed by 

 Mr. Ives, with 5,000 lines to the inch, is in everyway peculiarly adapted for 

 comparison. If such a grating is placed in a cone of somewhat divergent 

 white light from a lens, an interesting succession of colors appears when the 

 axis of the cone intersects a white screen one or more meters off. When the 

 grating is near the vertex, the axis is white; as the distance between grating 

 and vertex gradually increases, the corona shrinks and eventually a colored 

 central disk appears. The reds are vague; but the green, blue, and violet 

 disks, seen in succession, are very strong, each corresponding to a particular 

 distance of the screen. The close agreement of this occurrence with the disk 

 colors of coronas is striking and seemed worth further investigation. It is 

 necessary that the whole grating be illuminated, as the edges are largely 

 responsible for the phenomenon, while the successive annular regions modify 

 it. Hence, an annular method of illumination suggests itself, the annulus 

 being concentric with the circular rulings. In case of sunlight, a breadth of 

 annulus of about i mm. for the diameter of several centimeters gave good 

 results. 



