REFKACTION OF LIGHT, HEFRACTOME I RY AM) INTERFEROMETRY 



Lob and Pfund Refractometers. The ferences. A Nichols refractometer placed on 

 principle of the Lob refractometer is identi- the stage of such a modified microscope then 

 cal with that of the Pulfrich refractometer, becomes a small interferometer, the beam 

 but all the readings are made with the same spacing of which depends upon the refractive 

 telescope used for indicating the direction of index of the substance under study, 

 critical emergence. The eyepiece is fixed, For work in the ultraviolet, M. M. Hilger 

 and it includes an accessory prism for com- manufactures a photographic angle refrac- 

 pensating the spectral dispersion. Instead of tometer which appears to be but a commer- 

 the Pulfrich prism, a double Abbe prism is cial version of an instrument originally de- 

 used, signed by Charles Henri (34). 



The Pfund refractometer utilizes the The chemical micro-refractometer of Jel- 



principle of the total reflection of light at ley (31) is intended for the rapid determina- 



normal incidence at the boundary of two tion of the refractive index of immersion 



different media. A convenient instrument of liquids used for the identification of crystals 



this type for the study of the solidification by one of the immersion methods employing 



process of varnishes and lacquers has been the Becke lines or the Christiansen effect, 



described by Countryman and Kunerth (30). The essential part of the instrument is a 



Other Prismatic Refractometers. An small glass wedge of refractive index rig , sup- 

 early modification of the Abb6 refractometer ported by a plate. The wedge supports a 

 was the Fery instrument, already mentioned, drop of the specimen, forming a liquid prism 

 In this instrument the curved faces introduce of index n^ and of refracting angle a. This 

 "caustics" which produce greater uncertain- prism is about 1 mm thick. A narrow slit is 

 ties on the readings. Despite this defect, this adjusted so that a thin beam of light falls 

 instrument has been used for an extraor- upon the prism about the middle of the 

 dinarily large volume of data relative to or- wedge. An observer sees a sharp virtual 

 ganic chemistry. image of the slit formed upon the scale which 



Cruikshank and Fairweather (31) have is graduated directly in terms of refractive 



modified the Fery refractometer to adapt it indices, 



for continuous recording. The instrument can also be used con- 



The Nichols (32) double reflection micro- jointly with a heating device for measuring 



scope attachment for refraction measure- the refractive indices of melted substances, 



ments in reahty must be classified with the up to about 300°C. 



prismatic refractometers. It has been The Jelley refractometer is particularly 



adapted to microanalytical work by Alber well adapted to chemical studies when a 



and Bryant (33). This instrument, requiring modest sensitivity is required. Its commer- 



only 5 to 10 mm of fluid, may be operated cial version, the Jelley-Fisher refractometer, 



at high temperature (melting point refrac- described by Edwards and Otto (35), is cur- 



tivity) and is most convenient for chemical rently used to measure indices at the melt- 



studies. Its sensitivity can be considerably ing point. For measurements at still higher 



increased by using an interference method for temperatures (up to 300°C), one can use 



measuring the displacement of the two beams Fredriani's modification (36), which permits 



of light. It is possible to place a double simultaneous observation of the melting 



Young's slit over the conventional micro- points. 



scope condenser. If the aperture is limited to Broumberg's focal refraction monochroma- 



the production of the so-called marginal tor (45), although not using a prism, is 



rays, then the emergent light is coherent, based upon Snell-Descartes' formula, and 



and suitable for the production of inter- therefore it may be considered in the same 



490 



