EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 85 



which for x' = y = o, reduces to the above values. If % = %' and y = o, i.e., if a 

 colored ring coincides with the white ring, 



zDx 



When the red center is in adjustment, the distribution of colors along the 

 axis from screen to grating is naturally in the order of violet, blue, green, etc., 

 and they are relatively dull second-order effects. 



FIG. 46. 



FIG. 47. 



49. Experiments with small coronas. To reproduce these phenomena ob- 

 jectively with coronas is not difficult, so long as the coronas are of small 

 aperture, like those obtained from lycopodium spores. If the adjustment in 

 fig. 456 is used, the annulus should be sharp on the screen. The dusted plate 

 G may be placed anywhere near the focus F, showing, moreover, that the thick- 

 ness of the fog-chamber would not be effective. The best results were obtained 

 by using a pair of lenses of long focal distance, together, as in fig. 47, where 

 A is the annulus, L and L' lenses of focal distances 225 and 120 cm., respec- 

 tively, G the lycopodium plate, S the white screen at a distance of about 2 

 meters. By moving L and L' together or the reverse, i.e., by varying their 

 distance D apart, the diameter ww' of the white ring may be varied, while it 

 is kept in sharp focus. For the focal power is i/F = CC'D, C and C' being 

 constant. The white annulus should be about i mm. broad and 5 cm. in 

 diameter. Sunlight is preferable. If the ring is too thin, the colors are vague 

 from an insufficiency of light. 



