. Carnegie Institution of Washington publication. THE AID OF THE ACHROMATIC FRINGES. 35 16 It is a curious fact that although the ellipses can not be produced nor the slit much widened, apparently achromatic fringes do occur in white light for a micrometer placement at P such as should produce centered ellipses. Moreover, as the white slit-image is linear, the achromatic fringes are pref- erably made to run transverse to it. They are then exceedingly brilliant, extending much beyond the slit-image, and they travel up and down it with the motion of either micrometer at P or at n, (), or wit
. Carnegie Institution of Washington publication. THE AID OF THE ACHROMATIC FRINGES. 35 16 It is a curious fact that although the ellipses can not be produced nor the slit much widened, apparently achromatic fringes do occur in white light for a micrometer placement at P such as should produce centered ellipses. Moreover, as the white slit-image is linear, the achromatic fringes are pref- erably made to run transverse to it. They are then exceedingly brilliant, extending much beyond the slit-image, and they travel up and down it with the motion of either micrometer at P or at n, (), or with the rotation of the rail (Aa). As there are but four or six fringes with but one or two strong and brilliant, they make an exceedingly sensitive index for measurement. The occurrence of achromatic fringes may also be detected in the solar spec- trum, as all the Fraunhofer lines (homogeneous light) become helical and broad from the cross-hatching due to the fringes. Here with homogen- eous light the fringes are indefinite in number and follow each other contin- uously, whereas with white light but one or two intense black-white fringes appear. Though the achromatic fringes are by far the most brilliant part of the phenomenon, they rarely occur without streamers. The general appearance is roughly suggested in figure 16, where ss is the white slit-image in the telescope and a the achromatic fringes moving up and down 55 when or A« change. In the lateral glare of the field, however, fan- shaped or radiating coarse fringes bb are seen, intersected with very fine hairlike fringes cc. Probably there is also an intermediate group. These streamers are very useful to register the approach of the achromatic fringes, which move so rapidly that they are easily lost. A few measurements or rather estimates were made to coordinate the values of of the micrometer displacement at n and the corresponding rotation Aa of the rail necessary to annul this displacement. To do this the ach
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