120 THE INTERFEROMETRY OF 



63. Broad slit interferences. Achromatic fringes. Some allusion has been 

 made above to a type of interferences totally different in size from the regular 

 fringes and seen in the broadened slit. These were finally isolated and show 

 exceedingly interesting properties. They appear to best advantage, in the 

 absence of the spectroscope, in the broad white field of a very wide slit. The 

 latter may be removed. They have the appearance when vertical of regular 

 Young or Fresnellian fringes, very sharp and fine, achromatically black and 

 white at the middle of the grid, colored and fainter outward. They are 

 vertical when the enormously larger spectrum fringes discussed above are 

 centered. Like these, they partake of displacement here through the broad 

 white slit image, and this displacement is extremely sensitive in relation to 

 the displacement of the opaque mirror M' (fig. 73) to which it is due. Thus 

 a displacement of AN = icr* cm. of the latter corresponds to a march of fringes 

 through about 0.017 f the telescopic field of 3; i.e., to 0.05. This comprises 

 two fringes or AN = $Xio~ 5 cm. per fringe. Now, these fringes are so 

 sharp and luminous that it should be possible on proper magnification to 

 measure a few hundredths of this with an ocular micrometer. It is from this 

 point of view that I regard the new fringes important. They supply the fine 

 fiducial mark in displacement interferometry for which I have long been 

 seeking. They appear in a white field, thus requiring no spectrum resolution 

 nor monochromatic light. Moreover, the source of light need not be intense. 



To have a distinctive name for these fringes which will be much used in the 

 work following, I shall refer to them under the term "achromatic fringes." If 

 not too large, the central fringes are straight and almost quite black and white. 



The displacement of fringes with AN at the mirror (when n\ 2 AN cos 6) 

 is so rapid that if they are lost it is difficult to find them, unless the centered 

 large spectrum fringes in the spectroscope are first reestablished. The latter 

 are easily found. A removal of the prism grating g, figure 73, and a widening 

 of the slit show the achromatic fringes. The datum for sensitiveness may 

 be found directly as follows: The displacement at the mirrors corresponds 

 to about two residual fringes. Thus a single fringe (distance apart of the 

 intensely black lines at the center which can be distinguished and used as 

 fiducial lines for this very reason) corresponds to a displacement of mirror 

 of AA/ r = 5oXio~ 6 , as above. The white pattern, as a rule, appears but once 

 and is not usually present rhythmically, as is the phenomenon in the next 

 section for homogeneous light. 



As a clue to the nature of the residual fringes, one may note in the first 

 place that they may be recovered in the principal focal plane, if the two white 

 slit images which may have separated be put in coincidence. Without such 

 coincidence they are seen sharply in other focal planes. 



Later, on more careful adjustment as to parallelism by the auxiliary normal 

 method described below, periodic reappearance of the achromatic fringes 

 was in fact obtained. The central set was exceedingly strong and sharp as 

 usual. To the right and left of it similar patterns or groups rapidly decreasing 

 in strength were discovered. Not more than two patterns on each side of the 



