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PHYSICS: C. BARUS 
Further work was also done with sunlight to obtain more pronounced 
achromatics. For this purpose a compensator, C, was inserted to equalize 
the glass path in the half silvered plate. Huge spectrum ellipses were obtained 
in this way and their centers were placed above the telescopic field so that 
the fringes seen were large horizontal bars. On removing the spectroscope 
and placing the slit images out of focus, brilliant achromatics were now ob- 
tained, of the concentric hyperbolic type, vividly colored and broad between 
the apices, and diminishing to hair lines laterally. With these it was possible 
to enlarge the slit to at least 3 mm., without destroying the fringes, though 
they became more vague. It is again necessary that the slit images, when in 
focus, should be quite parallel. It was possible to place a plate of ground glass 
on the far side of the slit, without destroying the fringes; but not on the side 
towards the interferometer. In other respects the behavior was as described 
in the case of achromatics in the earlier experiments with a cleavage prism. 
Finally the spectrum fringes and the corresponding achromatics were ob- 
tained with the light of a Nernst filament, by focussing an image of it with a 
strong condenser lens on the slit. The experiments however are very difficult. 
The spectrum fringes are often weak, out of focus and extremely sensitive to 
small disadjustments in the horizontal and vertical coincidence of the slit 
images. They require a fine slit. When well produced the achromatics are 
also obtainable on removing the spectroscope when the spectrum fringes are 
horizontal bars. The achromatics may also be obtained brilliantly without 
the condenser lens; but the adjustment must in such a case be made first with 
sunlight, as the spectrum from the Nernst filament is too feeble for detecting 
fringes so elusive as the present. The achromatics however are strong and 
brilliant even here (Nernst filament). 
An interesting result is obtained in case of the achromatic fringes by narrow- 
ing one of the beams, for instance that coming from the mirror m (figure 1), by a 
screen with a slit about 2 mm. wide. In such a case the slit image, out of 
focus, is correspondingly narrowed. It may be passed from side to side of 
the broad washed slit image coming from the mirror n, by moving its adjust- 
ment screws (vertical axis). The fringes then appear only in a particular 
position of the narrow image in the field of the broader; but when they do 
appear they spread far beyond the margins of the narrow image on both sides. 
Interference thus apparently occurs where but one beam is present. The 
phenomenon is like those instanced before and means, as I understand it, 
that the beams have met in some other focal plane, though one is tempted to 
conclude that interference is stimulated by resonance, in particular as it is 
often impossible to find a plane in which they have met. The achromatics 
may sometimes be seen before and behind the principal focal plane, but more 
frequently either in the one or in the other region, only. 
Advance note from a Report to the Carnegie Institution of Washington. 
2 These Proceedings, 3, 1917 (565). 
