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7.0 NOTE ABOUT THE MEAN DISPERSION VALUE
Example:
Calculating the mean dispersion of distilled water
The refractive index, nD = 1.3330 at 20° C
The readings taken from the dispersion scale are:
Rotating in one direction Rotating in the opposite direction
41.7 41.5
41.6 41.6
41.6 41.6
41.6 41.7
41.7 41.6
The mean values are: 41.64 and 41.6 and the mean values of these give Z = 41.62
Looking in Table 1 interpolating when nD = 1.3330 we nd that
A=0.0248 15 and B = 0.033056
When Z = 41.62
0 = -0.5716 (because Z>30, 0 will have negative sign (-))
The mean dispersion value of distilled water can now be calculated according to the formula
(see Table 1):
nF - nc = A + B*0 = 0.024815 - 0.033056 x 0.5716 = 0.00592
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