1895.] MICROSCOPICAL JOURNAL. 171 



section-plates, are so close together that it was consid- 

 ered advisable to ascertain in some other manner whether 

 the order of dispersion was trnly represented ; that is, 

 whether the angle for one end of the spectrum was 

 really very slightly greater than that for the other end, 

 or whether the amount of dispersion thus indicated did 

 not really fall within the limits of experimental error, 

 thus leaving it possible that the dispersion might even 

 be of the contrary order. By immersing a plate perpen- 

 dicular to the first medium line in a liquid of refractive 

 power equal to the medium refractive index of the crys- 

 tal, the interference figure in white light usually at once 

 indicates, by the colors exhibited on the margins of the 

 axial brushes, the order of dispersion, and measurements 

 of the axial angle for the two extreme wave-lengths 

 afford an imn:iediate check upon the accuracy of the cal- 

 culated angles. It is a considerable source of satisfac- 

 tion to be able to comfirm such calculated optic axial 

 angles in so simple a manner. 



Prof. Klein further describes how admirably the new 

 apparatus is adapted for the determination of the ex- 

 tinction angles upon the various faces of a zone, in par- 

 allel polarized liglit. For this purpose the converging 

 lenses are removed, and the eye piece analyzing nicol is 

 employed, so that the polarizing and analyzing nicols 

 may be arranged for simultaneous rotation. The meas- 

 urements are carried out while the crystal is immersed 

 in the liquid as in case of the determinations of optic 

 axial angle. The only precaution necessary is that the 

 crystal should be uniformly illuminated in order that the 

 exact position of extinction may be ascertained by use 

 of one of the usual half-sliadow stauroscopic plates. 



The memoir concludes with a description of the gen- 

 eral mode of investigating a biaxial crystal immersed in 

 a liquid of equal refractive power, indicating how the 



