REFRACTION OF LIGHT, REFR ACTOMETRY AND INTERFEROMETRY 



where the n's are the refractive indices suc- 

 cessively measured with the hght from the 

 Z)Na hne of sodium, the Ca line of hydrogen, 

 and the F^ line of hydrogen, respectively. 



For this measurement, the two Amici 

 prisms of the refractometer are mounted in 

 such a way that they can be rotated in op- 

 posite directions about the optical axis of the 

 telescope, their respective positions being 

 read on an arbitrary scale. 



With all types of critical angle refractom- 

 eters, certain factors limiting the accuracy 

 of the measurements are not entirely con- 

 trollable by the operator. These factors are: 



(1) The extreme temperature sensitivity 

 of the instrument, producing a hazy, shift- 

 ing, dividing line, even when a thermostatic 

 jacket is provided. 



(2) The difficulty in setting the exact in- 

 cidence and proper convergence for the light. 

 This factor affects the contrast at the divid- 

 ing line in the field of the telescope and 

 limits the reproducibility of the settings. 



(3) The limited power of the dispersion 

 compensation prisms. These prisms are effi- 

 cient only for the D lines of sodium light, 

 and they exactly correct images that are 

 formed only near the center of the field. 

 With liquids of high refractive indices (oils, 

 etc.) the boundary is always somewhat 

 broad and iridescent. 



(4) The accuracy on the reading of the 

 divided circle (critical angle). This last 

 point is discussed in physics textbooks. 



Some of these difficulties are reduced by 

 designing refractometers covering certain 

 limited ranges of indices, for special indus- 

 trial applications (oils, sugars, resins, wines, 

 etc.), and by using readily interchangeable 

 monochromatic sources of radiant energy, 

 now commercially available at reasonable 

 price and directly operated from the power 

 line (filtered carbon arc, mercury arc, helium 

 and hydrogen tubes, Na, Sr, K, Cd, Kr, Ne, 

 lamps, etc.). A convenient table for the se- 

 lection of suitable radiation sources will be 



found in an article bv Tilton and Taylor 

 (79). 



Microscopic Methods Based on 

 Change of Focus. The method of deter- 

 mination of the true thickness of an im- 

 mersed object by means of the microscope 

 was originally invented by the Due de Chaul- 

 nes, and was completely discussed by Jo- 

 hannsen (53), the Winchells (54), Addey 

 (63), Blunck (65), and summarized by Cha- 

 mot and Mason (58). This method is very 

 widely used in chemistry. 



A glass plate is provided with a hollow 

 cell 0.5 to 2.0 mm deep and about 3 mm in 

 diameter. The bottom of the cell is polished 

 with parallel faces and scratched. The cell 

 is filled with a liquid to be studied and cov- 

 ered with a cover glass, itself scratched. The 

 apparatus is placed under the microscope, 

 and the apparent vertical distance between 

 the virtual images of the scratches is meas- 

 ured with the micrometric adjustment of the 

 microscope. The true distance between the 

 two faces of the cell is also measured in the 

 absence of liquid in the cell (n = 1). The 

 refractive index of the liquid is given by the 

 relation : 



n = true thickness/apparent thickness (H) 



The micrometric adjustment of the micro- 

 scope can be calibrated directly in terms of 

 refractive indices by the use of liquids of 

 known refraction. 



When the solid specimen available is too 

 small (powders), the above mentioned crys- 

 tallographic methods cannot be applied. It 

 is, however, possible to evaluate the refrac- 

 tive index of these solid specimens by immer- 

 sion in liciuids of convenient refractive index. 

 When the index of the immersion liquid 

 equals that of the immersed specimen usually 

 some sharp modification of the microscopic 

 image of the latter can be observed. The re- 

 fractive index of the liquid is then measured 

 by one of the methods described. The equal- 

 ity of the indices can be ascertained by the 

 microscopical observation of either one of 



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