REFKACTION OF LIGHT, REFR ACTOMETRY AND INTERFEROMETRY 



of microscopy. They were critically reviewed prism which also supports the specimen un- 



by Say lor (47). The utilization of the Chris- der investigation. A suitable telescope is 



tiansen effect falls into the same category, placed on the small side of the prism, near 



In general, such methods have been brought its right angle. This telescope is mobile 



to a high degree of perfection mostly for the around an axis perpendicular to the plan of 



requirements of petrographic studies (48 refraction of the rays of light, and centered 



49, 50). They culminated in the very perfect around the right angle of the prism. It is 



microscope stage for crystallography de- attached to a graduated circle, so that the 



signed by Emmons and Winchell (39). The angle of emergence of the refracted rays can 



measurements can be extended to any sub- be measured. When the direction of emer- 



stance exhibiting metallic reflection (51), as gence is found, the field of the telescope ap- 



well as to plastics and to resins (52). The pears divided into two zones: one bright, 



basic procedures remain those of earlier in- and one completely dark if the incident 



vestigators (53, 54, 55, 56, 57) and others, light is monochromatic. 



Several of these procedures can be combined The original Abbe refractometer was im- 



with the "immersion method" familiar to proved to permit measurements on both 



microscopists (46, 58, 59). The Merwin transparent and opaque liquids as well as on 



method for crystallography has long been solid specimens. To the hypotenuse face of 



standard. Those of Wood and Ayliffe (60), the original Abbe prism a second prism ex- 



and of Wood, Ayliffe and Williams (61) are actly identical is attached. The hypotenuse 



rather general for all solid specimens. face of this second prism, in contact with that 



The procedure of Kienle and Stearns (62) of the former, is finely ground and is provided 



for computation of refractive indices of with a small cylindrical cavity where the 



opaque specimens from iso-reflectance dia- specimen under investigation is placed. To 



grams and the Fresnel's foraiulas are illus- this combination is added a trapezoidal piece 



trative of the use of a spectrophotometer of glass of such a shape that the incident 



(American Cyanamid Co. automatic re- ray of light falls upon the surface of the 



corder) for such a purpose. specimen. 



Refractometric measurements by means The methods available for measuring the 



of the microscope are considered by Addey characteristic constants of the critical angle 



(63). refractometer are often successfully used in 



The Critical Angle Refractometer: setting up other types of optical instruments. 

 Abbe Instrument. Measuring instruments such as spectrometers. The original Abbe 

 of this type are based upon Snell's formula refractometer has been modified and im- 

 (1). The measurement consists in observing proved in numerous ways by ingenious 

 the critical angle of emergence Vc with light manufacturers. Precision type instruments 

 falling at grazing incidence {i practically are currently manufactured in this country 

 equal to 90°) upon the boundary of two by Bausch & Lomb Optical Co., American 

 transparent media, one of which has a known Optical Co., Precision Scientific Co., In- 

 refractive index, rig . dustro — Scientific Co., and by the Gaertner 



The simplest instrument of this category Co. The latter, originally developed by Ja- 



is the Abbe refractometer. It is essentially cobson is suitable for turbid fluids. The 



formed of a flint prism whose angles are re- Ostwald refractometer (Zeiss Mfg.) is a dif- 



spectively 90, 60, and 30 degrees. A slightly ferential instrument based upon the same 



convergent beam of light is furnished by a principle. 



suitable condenser and falls at grazing in- The Fery Refractometer. A different 



cidence upon the hypotenuse face of the version of the application of the principle of 



488 



