16 CONDENSATION OF VAPOR AS INDUCED BY NUCLEI AHD IONS. 



TABLE 6. Data for axial colors. ra = 3.6X io~ 8 grams; v = i.9oXio~ 2 . 



17. The lamellar grating. Recently I have considered the case of 

 coronas in relation to the lamellar grating, in which diffractions are 

 obtained from a uniform succession of alternately different thicknesses 

 of clear glass. Experiments with such gratings were originally made 

 by Qiiincke and there is a full theoretical treatment by Verdet. The 

 behavior of this grating differs from that of the usual kind in the occur- 

 rence of an additional factor 



cos*(7cd(n i) + Tea sin o) jX 



where n is the index of refraction, d the difference in thickness between 

 thin strips of width a and thick strips of width b, a the angle of dif- 

 I r^.ction. Hence since for axial color, = 0, minima occur at ( i)J = 

 + i) .XI 2, whereas for Newton's interferences, the mimima occur for 

 a thickness D in the case of transmitted light where 2 ;j.D= (2111+ i) .XI 2; 

 whence 



d/D = -^- 



n i 



In case of water ;/ = 0.133, or d/D = 8.o. This result applied to a grating 

 of transparent strips is so near the above datum d/D>6 for a medium 

 of transparent particles (for which there is no theory), that it seems 

 reasonable to conclude that the actual colors are referable to the same 

 type of phenomenon in both cases. The need of observations through 

 long columns in case of fog particles suspended in air is additionally con- 

 firmative, since the contribution of color due to one particle must be 

 exceedingly small. 



18. Disk colors of coronas. One might be tempted to explain the 

 disk colors in the same way, for in case of deviation d from the axial ray 



D I d= (n i)/2W + a sin 0/2 dn 



But here there are several insuperable difficulties which refer the disk 

 color to a different origin. In the first place, they are much more intense 



