PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volumes MARCH 15. 1919 Number 3 
NOTE ON THE SELF-ADJUSTING INTERFEROMETER IN 
RELATION TO THE ACHROMATIC FRINGES 
By Carl Barus 
Department of Physics, Brown University 
Communicated, January 3, 1919 
1. Introductory. — This apparatus was first used for the case of coincident 
ray systems by Michelson and Morley^ in their famous experiments on the 
Fresnel-Fizeau coefficient and has since been similarly employed by Zeeman.^ 
It has so many practical advantages that a special reference here is 
justified. It is shown in a modified form^ in figure 1, where L is a pencil of 
white light preferably from a collimator. It is separated by the half silvered 
plate N into the two beams LI 2345 T and L16785T, both of which are recom- 
bined at 5 and then enter the telescope at T, It is merely necessary to rotate 
any of the mirrors, say iV, around a vertical axis until the two vertical white 
wide slit images coincide in the telescope, when brilliant fringes will be at once 
obtained on the coincident white fields. The central fringe is achromatic, 
for the system is self-compensating, or the glass paths are rigorously equal. 
The fringes may be enlarged to infinite size and then reduced in size again, 
the phenomenon being symmetrical, by rotating any mirror, say iV, about a 
horizontal axis. 
The mirrors N and N' are rigidly fixed to a carriage capable of sliding right 
and left parallel to the lines 85, etc. Hence the rays 85 and 21 and the rays 
34 and 76 may be brought to coincidence or separated in any degree, at pleas- 
ure. If the slide were perfect the fringes would not be disturbed by this 
process; but few slides are perfect to this degree, and the fringes will change 
size somewhat, since there is rotation. Practically this is of no consequence. 
The fringes, which when sharp are necessarily horizontal, may also be 
changed in size by inserting a plate glass compensator about 5 mm. thick in 
the paired beams 85, 21, or 34, 76. When this is rotated on a horizontal 
axis, the fringes pass through infinite size, and this arrangement is particu- 
larly adapted for the detection of the character of the fringes and will be so 
used presently. 
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