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PHYSICS: A. A. MICHELSON 
In either case, when the adjustment is perfect, the fringes are concentric 
circles — which remain constant when the eye or the observing telescope is 
moved about in any direction, if the grating is perfect; and if not, measurement 
of the diameters of the circles gives the error. 
If, however, the difference of path in the interferometer is small, a curious 
singularity is presented. The interference fringes are no longer circles but 
complicated forms expressible by the formula: 
A = (y — y Q ) (x 2 + y 2 ) + ax + @y. 
Further details will appear in a coming number of the Astrophysical Journal. 
