598 WRIGHT: MEASUREMENT OF STRAIN IN GLASS 



constructed that the optical elongations of the two halves are 

 equal but of opposite sign, the result being that on the insertion 

 of a birefracting plate in the diagonal position the dark band in 

 the one-half moves to the right while in the second it moves to 

 the left as indicated in figure 1 c. With a properly constructed 

 bi-compensator the shift produced by a path difference of less 

 than Imm can be readily detected and measured. The sensitive- 

 ness of the combination wedge varies inversely with the pitch 

 of the wedge. The most favorable conditions for observation 

 are: intense illumination by parallel, monochromatic rays; long 

 wedge with very low pitch, such that 1 scale division (0.1 mm.) is 

 equivalent to a path difference of 1 nfx; magnifying system for 

 examining wedge located above analyser. This last precaution 

 does away with the disturbing rotatory effects on the planes of 

 vibration of the transmitted waves at the surfaces of the lens 

 system. For most purposes, however, such precautions are 

 not necessary and it is sufficient to use a less sensitive bi-com- 

 pensator (1 scale division = di^n or 10 nn) in the lower focal 

 plane of a positive eyepiece in a petrographic microscope, the 

 analyser then to be a cap-nicol. 



The conversion of the optical retardations thus measured on 

 a given piece of glass to the corresponding elastic deformation 

 data is accomplished by direct measurements on the glass sub- 

 jected to a series of mechanical loads of known magnitudes. 



The above optical method for measuring relative strain in 

 glass has been employed for several years in practical glass work 

 and has been found to be satisfactory." In certain instances 

 details of the method require modification to meet best the special 

 conditions of measurement, but such adaptations are readily 

 made and the general underlying principles remain unchanged. 



MINERALOGY. — Additional notes on babingtonite from Passaic 

 County, New Jersey. Clarence N. Fenner, Geophysical 

 Laboratory. 



In the November 19th issue of this Journal the writer de- 

 scribed an occurrence of the mineral babingtonite in the deposits 



" R. L. Frink. Eighth International Congress of Applied Chemistry, 5 : 13. 1912. 



