IQ2 ELEMENTARY CHEMICAL MICROSCOPY 



their respective indices of refraction and n the refractive index 

 of the mixture. It is obvious that in the preparation in this 

 manner, of liquids of intermediate index values, it is essential 

 that the two liquids shall be miscible in all proportions, and that 

 no new chemical compound shall result from the mixing. Since 

 a determination of refractive index may often require a period of 

 time sufficiently long to result in an appreciable loss of liquid 

 through evaporation, the liquids chosen for mixing should, 

 theoretically, not differ greatly in their boiling points, otherwise 

 there is a possibility of the concentration of the less volatile liquid 

 increasing. It will also be obvious that in order to obtain suffi- 

 ciently exact calculated values from the equation given above, 

 the liquids mixed should not have widely different densities. For 

 these reasons approximate boiling points and densities have been 

 given in Table I. This formula for calculating the index of refrac- 

 tion of a mixture of two liquids is based upon the assumption that 

 to the final mixture each component contributes equally its own 

 proportional part of the final index. There seems to be no sound 

 theory in support of this assumption nor do the facts appear to 

 be in accord with the formula. From experiments made in the 

 Cornell laboratories, using an Abbe refractometer to determine 

 the refractive index of the liquids before and after mixing, it was 

 found that the calculated results were not always dependable to 

 the second decimal place. The first decimal was always correct, 

 the second usually so, but very rarely indeed would the third 

 agree in calculated and found values. Mixtures allowed to 

 stand several days and again measured gave similar results, 

 showing that equilibrium had been reached when the first obser- 

 vations had been made. 



Formulas of the kind given above should not be employed if 

 the same degree of accuracy in calculation is wanted, as the im- 

 mersion method will yield in practice. 



The immersion method above described permits an accuracy 

 in the determination of the refractive index within 0.005 but 

 with monochromatic light and more refined methods of illumi- 

 nation an accuracy of 0.002 or even o.ooi db may sometimes 

 be reached. 



