ANGLE REFRACTOMETRY 



the critical angle to the measurement of re- ing (and approximately evaluating) the dis- 



fractive indices is embodied in the Fery re- persion. 



fractometer. In this instrument, the refract- Various attachments, which need not be 



ing prism is a hollow body to be filled with mentioned here, permit one to adapt the 



the liquid under investigation. The faces instrument for measurements on either bulk 



comprising the refracting angle are made liquids, single drops, properly cut trans- 



of two half -lenses. This body is mobile trans- parent solids, or opaque liquids, 



versely to the path of light, so that a narrow The accuracy of the measurement is of the 



beam of collimated light can be made to pass order of 0.00003 to 0.00004, according to the 



through it at various distances from its re- position of the dividing line in the field of the 



fracting angle a. A telescope provided with a telescope. 



cross-air reticule permits one to observe the The Pulfrich Refractometer. This de- 

 apparent position of another reticule dis- vice utilizes the total reflection principle. A 

 posed inside the collimator tube. This ap- glass prism of high refractive index has two 

 parent position depends upon the refractivity polished faces perpendicular to one another, 

 of the liquid contained in the prism. The one of them being horizontal. This horizon- 

 image of the collimator reticule is brought tal face has a small glass cell cemented to it 

 back in coincidence with that of the reticule for retaining the liquids to be examined, 

 of the telescope by moving the prism by The beam of incident light is directed so as 

 means of a micrometric screw. This displace- to strike the prism at grazing incidence after 

 ment is a simple function of the refractive in- traversing the liquid. The emergent beam 

 dex of the liquid. passes through the vertical face of the prism. 



This instrrmient really makes use of a It is sharply outlined by those rays which 



"variable refraction angle prism." It has actually graze the prism surface. The sharp 



known a very prolonged success in chemistry, boundary is observed with a telescope at- 



probably because of its low cost of construe- tached to a divided circle and vernier. Read- 



tion and its ease of operation, but is less ings are possible to 6 seconds of arc. For 



satisfactory than the Abbe refractometer. accurate temperature control, the optical 



The Dipping Refractometer. The "dip- parts of the instrument are surrounded by 



ping refractometer" has received very wide a water jacket. 



acceptance in industry. Since it can be ap- The instrument is intended for measure- 

 plied to measurements even in flowing fluids, ments on liquids, the refractive indices of 

 it is the nearest thing, in its class, to a truly which are lower than that of the glass of the 

 industrial apparatus. prism. To extend the range of measurements, 



The dipping refractometer is designed on interchangeable prisms of different indices 



the principle of the Abbe refractometer. The are available. The accuracy on the refractive 



refracting prism is similar to the upper index of liquids is about 0.00001. It reaches 



prism of the Abbe apparatus, but there are 0.00002 on the measurement of the dis- 



provided usually six interchangeable prisms persion with monochromatic lights, 



of various indices covering the range of Solid samples must be cut so as to present 



measurements from 1.32539 to 1.54409. two perpendicular faces meeting at an un- 



The telescope is of conventional design broken line, while one of the faces must be 

 butof longer focus. The dividing hne between fairly well polished. Optical contact is ob- 

 the two halves of the field is formed in the tained in the usual way. 

 plane of a divided scale and its position is For convenience, the manufacturers sup- 

 read directly; no moving parts are involved, ply tables giving the value of n vs. the meas- 

 A single Amici prism is provided for correct- ured angle i, for various spectral lines. 



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