84 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



appear strongly in the center of the figure; i.e., in the center of a white non- 

 diffracted ring. If y is practically zero, these colors will at first sight seem to 

 be axial colors and would appear to be equivalent to those of the coronas of 

 the fog-chamber. This does not, however, seem to be the case, for reasons 

 presently to be given. 



TABLE 12. Disk colors of an annular grating. >=5Xicr 4 cm.; 

 R +^" = 700 cm.; X=o.7i4* (iy/x).+ 



It follows from equations (5) and (6) that, since D and R are given, X varies 

 as x, the radius of the white ring; therefore also with y, the corresponding 

 radius of the white ring at the grating, if the position of the grating is fixed. 

 For x/R = y/R", nearly. Hence, if the outer rings contribute a red center, 

 the inner rings would contribute a blue center, the superposition of all colors 

 being white light; i.e., the usual white disk of these grating coronas. On the 

 other hand, if the outer rings contribute a corona, the inner rings can only 

 contribute blues and violets, for there are no other first-order colors. Hence, 

 greenish-blue and violet centers may be produced from the grating as a whole, 

 without an annular source of light (as is in marked degree the case), whereas 

 the red axial colors occur only when the source is annular. 



It is interesting to note that when the central color is violet the reds have 

 already overstepped the center and are approaching the white ring. Fig. 46 

 shows that when the divergence of the undiffracted white annulus is not too 

 great, it is easy to produce the internal and the external annular spectrum on 

 the outside of the white annulus. In such a case, x' denoting the distance of 

 the red annulus from the center of the screen S, at a distance R from the 

 grating G, 



*-y __L_ _*J2 m * 



