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water used as bath liquid possesses a very small index of refraction. 
For the substances examined by us we have e.g. at 20°: 
tetrachlorethane  : np = 1.496 ; 
benzene nps DO 
whereas for water: np = 1.333 
lt is easily seen that on use of such solvents with greatly deviating 
indices of refraction, an exceedingly small departure from the parallel 
position of the plane-parallel front and back wall of the vessels, 
brings about a very great displacement of the upper interference 
image. Let us call the index of refraction of the bath liquid 7, 
that of the liquid in the vessel n,; let further the path in the bath- 
liquid (ab + cd in fig. 1) passed over by the beam of light which 
passes C,, be /,, that in the vessel /,. The optical path is then for 
vessel Ci, the beam passing over the path twice’): 
2(n,/, +n, 4). 
With perfect parallelism of the plane-parallel plates, this path has 
the same length for the other beam. If, however, the length of 
vessel (', is e.g. d more than that of C,, the optical path is here: 
2 [n, (ld) + n, (l, + od}. 
Accordingly the difference in optical path is: 
A = 2d (n,—n,), 
and the displacement of the interference image brought about by 
this and expressed in bands: 
A 4d (n,—n,) 
N=-= ; 
A À 
if 4 represenis the wave-length of the light used. 
In the case of water and tetrachlorethane n‚—n, is = 0.16. 
From this follows for 4 = 0.00058 mm. : 
N = 550 d, or expressed in scalar divisions: 
N'= 21 X 550 d = 11550 d sealar divisions. 
If the deviation from the parallel position d is e.g. 0.001 mm., 
the shifting amounts to no less than + 12 scalar divisions. The 
displacement observed with the vessel used by us appeared to be 
about 40 scalar divisions. Besides this displacement also the imperfect 
form of the upper image is partly due to the non-parallelism of 
front and back wall of the vessels. 
5. To obviate this difficulty, a liquid must be chosen as ‘‘Tempe- 
1) We may disregard here the thickness of the plane-parallel plates, the result 
not being affected by this, 
