PFIYSICS: C. BARUS 
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reddish and faint) in the rectangular or other interferometer, has been referred 
to in my earlier papers. As a rule the fringes found are more or less diffuse, 
non-symmetric, with large numbers (10 or more) about equally strong. Such 
fringes are, of course, useless in displacement interferometry. When the 
sharp fringes needed are obtained, their definition is independent of the par- 
ticular part of any of the glass plates used, and any plate may be rotated 180° 
in its own plane without spoiling the sharpness of the fringes. Hence such 
slight curvatures or wedge shapes as the plates may possess, are without 
influence on the phenomenon. To further test this I devised a screw press 
adapted to push the vertical edges of a plate to the rear and the middle forward, 
so as to give the plate marked cylindricity. Quarter and eighth inch plates 
were operated on, in the latter case sufficiently to give the two superposed 
slit images quite unequal width; but no essential or useful improvement of 
the fringes was observed. The type of the fringe was not altered. Again the 
symmetrical fringes may be obtained from plates thickly or thinly silvered, 
without essential difference. 
It follows therefore, that the relative thickness of the glass paths traversed 
by the interfering beams can alone be of influence in shaping the fringe pattern 
in the manner in question. This is in consonance with the general theory of 
achromatic fringes, the result being a superposition of the color phenomena 
due to the dispersive refraction of the glass and the colors resulting from the 
wave lengths of the interfering rays. To test this the annexed form of appa- 
ratus (see figure) is particularly convenient, as the fringes are easily found. 
Moreover both rays, a and c, from the collimator at L, eventually pass through 
the plate, N', before reaching the telescope at T. It is thus merely the thick- 
ness of the half silvers, M and N, both at 45°, that is here in question. If this 
thickness is the same, the sharp symmetrical design of but two strong fringes 
appears. If the difference of thickness is but little over half a millimeter, 
many fringes, non-symmetric in distribution are the rule. If the differential 
thickness is several millimeters there may be hundreds of fringes. If these 
are small they may be enlarged at pleasure; but they are always faint and use- 
less for measurement. 
1 Advance note from a Report to the Carnegie Institution of Washington. 
