. Carnegie Institution of Washington publication. EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 87 When an annular source is used, the path of the cone or cylinder of light through the fog is highly illuminated and the diffraction is both inward and outward from this shell. The shadow of the disk of the annulus, therefore, necessarily remains permanently dark, so that this should be small and the cone as divergent as the fog-chamber permits. The colors are quite brilliant, in spite of the fact that fewer particles are used than in the case of an unob- structed point source. In fig. 48, if
. Carnegie Institution of Washington publication. EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 87 When an annular source is used, the path of the cone or cylinder of light through the fog is highly illuminated and the diffraction is both inward and outward from this shell. The shadow of the disk of the annulus, therefore, necessarily remains permanently dark, so that this should be small and the cone as divergent as the fog-chamber permits. The colors are quite brilliant, in spite of the fact that fewer particles are used than in the case of an unob- structed point source. In fig. 48, if FF is a fog-chamber, ww and w'w' elements of the conical shell of light, a position of the eye of E behind the fog-chamber may be found, at which the whole interior of the cone flashes into the uniform color of the disk. For this purpose i must be small or zero, or the shell nearly cylindric, in which case, if R is the distance of the eye from the center of the chamber, s/R the aperture of a given color minimum, the elementary equation becomes In this case 5 is constant and R is variable. It is found difficult to use this method practically. An interesting accompaniment of these experiments is the occurrence of vivid color when the eye is in a proper position on the illuminated side of the fog-chamber, as, for instance, at E' or E". In other words, there is also vivid diffraction in connection with the reflected light, a phenomenon which it is difficult to detect in case of the absence of the annular screen A, since all the fog is illuminated. These colors come from both the outside and the inside of the cone. When the colors fade, or when there is much directly reflected light, they may be restored by a Nicols prism, as this on orientation cuts off the reflected light only. The diffracted colors are in no case polarized, whether seen by transmitted or reflected c/l FIG. 48. Experiments made at some length with these annular sources, supplied with both polarized an
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