ANGLE REFRACTOMETRY 



light as the prismatic refractometers. In fact, tained by auto-collimation. The two-dimen- 



it can be used for refractometric measure- sional plot may be directly projected on a 



ments. screen, or it may be magnified. 



Extensive data relative to the refractivity The Grauer angle refractometer is an in- 



of organic substances and the application of teresting adaptation of a two-prism system 



such measurement methods to the problems employed at the National Bureau of Stand- 



of organic analysis have been collected (64- ards for the study of solid specimens (76). 



68). Applications of refractometry to chemi- It could be easily adapted to the study of 



cal production control became really prac- fluids. 



tical with the advent of rugged recording "The instrument consists of a light source 



types of instruments. One of the early de- with a horizontal filament, a collimator, a 



signs is the multiple reflection instrument of diaphragm, a pair of 45-degree-90-degree 



Karrer and Orr (38), now obsolete. It was cemented prisms forming a 90-degree hol- 



based upon the principle of Fery's hollow low at the upper half and a parallel plate 



prism. The Barnes recording refractometer at the lower half, a filar micrometer, and a 



(41) is now superseded by the commonly telescope. All the components are centrally 



used industrial instrument of Barstow (40). aligned along a common axis." 



At present there is a tendency to extend Compensation of the Spectral Dis- 



a renewed interest in differential types of in- persion. In the preceding discussion it was 



struments. A differential refractometer prism assumed that the incident light was mono- 



is described by Kegeles (69). It consists of chromatic. With composite light, white or 



two identical, hollow rectangular prisms of variously colored, the critical angle of refrac- 



refracting angle A adjacent on their hypot- tion varies with the wavelength. If such 



enuse faces. One compartment is used for a light is used for the measurements, the 



reference fluid of index no , the other re- boundary line seen in the telescope appears 



ceives the test solution of index n. Parallel as a hazy colored band, the colors being ar- 



light enters normal to one of the small faces, ranged symmetrically in the two halves of 



passing through the test fluid first, and is the field. The dispersion of the light in the 



deflected from the normal by an angle given instrmnent is due to the combined action 



by Snell-Descartes' law. Similar prismatic of two factors: (a) the constant effect of the 



refractometers have been described by Tho- prisms, the physical dimensions of which are 



vert (70), Zuber (71), Longworth (72), Debye fixed, and (b) the variable effect of the speci- 



(73), and others. men under investigation. The first factor 



The advantage of such a design lies in its could be corrected once for all; the second re- 

 small size, so that sufficiently rugged meas- quires adjustment for every specimen in- 

 uring cells may be built for direct measure- vestigated. 



ment of sedimentation equilibrium in the In order to cancel out this phenomenon, 



ultracentrifuge (Kegeles (74)), by a modifi- the modern refractometers are provided with 



cation of Lamm's schlieren scale method two composed Amici prisms disposed inside 



(75), with the introduction of suitable cor- the telescope. These prisms are cut at a 



rections indicated by Kegeles. With suffi- suitable angle so that the light from the so- 



ciently high prismatic channels and suitable dium Dsa lines passes undeviated. 



optics, it is possible to observe directly a A rough estimation of the spectral refrac- 



two-dimensional refractive index distribu- five dispersion of substances under investi- 



tion in a heterogenous — say, for instance, re- gation can be derived from the conventional 



fraction increment dn, vs. a concentration expression: 



gradient. Greater deflections may be ob- d = no - l/Oip — nc) {lO) 



491 



