PHYSICS: C. BARUS 
363 
allusion has been made above to a type of interferences, totally differ- 
ent in size from the regular fringes, and seen in the broadened slit. 
These were finally isolated. They appear to best advantage in the 
absence of the spectroscope, in the broad white field of a very 
wide sHt. 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 ellipses discussed above are centered. Like these they par- 
take 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. 1) to which it is due. Thus a displacement 
of A = 10~^ cm. of the latter, corresponded to a march of fringes 
through about .017 of the telescopic field of 3°, i.e. to 0.05°. This 
comprises two fringes or A iV = 5 X 10~^ cm. per fringe. Now these 
fringes are so fine, sharp and luminous that it should be possible on 
proper magnification to measure a few hundredths of this with an ocu- 
lar micrometer. 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 monochromiatc 
light. Moreover the source of fight need not be intense. 
I shall in referring to the new fringes use the tenii residual or 
achromatic fringes. Their theoretical breadth should be A = 
X / 2 cos d agreeing substantially with the given estimate. 
The displacement of fringes with A N at the mirror (when n \ = 
A N cos 6) is so rapid that if they are lost it is difficult to find them, un- 
less the centered large fringes in the spectroscope are first reestablished. 
The latter are easily found. A removal of the prism grating gT, figure 
1, and widening of the slit shows the achromatic fringes. The white 
pattern usually appears but once and is rarely present rhythmically 
as in the case of the next section for homogeneous light. 
4. Wide Slit. Homogenous Light. Sodium Flame. — A clue to the 
nature of the residual fringes will be obtained when white light is re- 
placed by homogeneous light. A strong large sodium flame near the 
mirror M, figure 1, suffices. The fringes now appear of the same size 
in yellow light and naturally spread over a much larger area of field. 
But on moving the mirror (A N increasing continually) forward very 
gradually, the homogeneous fringes alternately vanish and reappear, 
each time however enlarged in size but still straight, until at an inter- 
mediate position of symmetry enormous round ovals cover the yellow 
field. The fringes then diminish symmetrically in the same way. The 
