KEFKACTIO.N OF LIGHT, KEFRACTOMETRY AND INTERFEROMETRY 



nall}^ led to the design of another type of re- nients embodied in the instruments hsted are 



fractomcter adapted to the study of opaque based also a number of methods which can be 



substances, namely, reflective refractome- carried out with such conventional instru- 



ters. The following account of this field of ments as the microscope, the goniometer and 



measurements is intended to serve as a guide, the photometer. These methods include: the 



In each case it is imperative to refer to the Becke lines displacement, the Christiansen 



original references, giving the exact detailed effect, the Merwin (46) and the Emmons 



procedure to be followed. (For references, re- methods for crystals, and the Wood and 



fer to article "Historical Introduction.") Ayliff method for solids in general. 



Transmission refractometers include: the Prismatic refractometers utilize either 

 Abbe simple and double types (26)* from one of the two basic properties of trans- 

 which are derived the various designs of parent media: (1) the existence of a mini- 

 "dipping refractometers": the Pulfrich re- mum deviation angle of emergent light, or 

 fractometer (27); the Lob instrument (28); (2) the existence of a critical angle, 

 the Pfund total reflexion device (29) and its Instruments based upon the determina- 

 modification by Countryman (30) ; the Ferry tion of the minimum deviation angle have 

 hollow prism refractometer; the Jelley re- only an historical interest. Their passing into 

 fractometer (31); the Jelley-Fischer (35) in- oblivion is perhaps unjustified, for it is now 

 strument adapted for observations on molten possible to measure — and to record or con- 

 substances ; the Nichols double reflection in- tinuously monitor — ^with them small angular 

 strument (32) and its modification for micro- deflections with an approximation better by 

 analysis (33), both of which must be used several decimal places than was possible half 

 conjointly with a microscope; the Hilger ul- a century ago. Such instruments, being 

 traviolet refractometer designed after the basically extremely simple, might be worth 

 original instrument of Ch. Henri (34); the reconsidering for certain industrial applica- 

 Fredriani instrument (36), really a modifica- tions. For these reasons, additional com- 

 tion of Jelley 's; the recording modification of ments on the goniometric method and of that 

 Ferry's design used by Cruikshank and Fair- of Biot and Arago are given, 

 weather (37) in their chemical work; the Methods of Newton, and of Biot and 

 photoelectric multiple reflection device of Arago. Only in rare instances is it possible 

 Karrer and Orr (38) ; the Emmons spherical to measure the angle of refraction directly, 

 refractometer for solids (39) ; the recording Such measurements, of limited accuracy, are 

 refractometer of Barstow (40) ; the "chemical convenient when working with properly 

 refractometer" of Barnes (41); the Jacob- shaped transparent solids. They are easily 

 sohn instrument for opalescent fluids (42) ; effected with a goniometer, sometimes called 

 the Stamm et al. differential recording in- a "spectrometer." The conventional goni- 

 strument (43), using either three Wernicke ometer is described in practically every text- 

 hollow prisms, or a double Zenger hollow book. More elaborate instruments of the 

 one; the Clamann pneumatic refractometer "transit" type, when equipped with reflec- 

 for gases (44), in which a beam of light is de- tion or internally illuminated auto-collima- 

 flected at the interface of two gas phases, tion devices, are usable for this purpose, 

 imparted with a parallel rapid laminar flow; In all cases the measurement method is 

 the Broumberg refraction monochromator, based vipon the property of a prism of ex- 

 which can be adapted to refraction measure- hibiting a minimum deviation angle. The 

 ments (45) ; and a few others mentioned else- existence of this angle is a direct consequence 

 where. of the critical refraction angle. The method 



On the principles of angular measure- originally designed by Newton, and by Biot 



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