ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 697 



emitted flames. Behind the slit is a nicol N, movable in and out, and a 

 plane parallel glass plate <J. inclined at 45°, and also movable in and out. 

 The nicol is used for observing Zeeman effect, electrical and magnetic 

 double refraction, normal and abnormal magnetic rotations. For observ- 

 ing the Zeeman effect parallel to the lines of force, a mica J-A double plate, 

 right and left circularly polarized, is placed in front of the slit ; the nicol 

 is then pushed in. For observing the same phenomenon perpendicular 

 to the lines of force there are two methods : one is to place in front of 

 half of the slit a plate of quartz cut perpendicularly to the axis ; the other 

 is to place a |-A double plate of mica. A Rowland's grating is then 

 applied in order to avoid the overlapping of the spectra. The nicol is 

 also so placed that its direction of displacement (electric or Fresnel's 

 victor) is vertical. For measuring electric or magnetic double refrac- 

 tion, the mica plate is brought on to the slit of the collimator (after 

 Koenigsberger) ; then comes the compensator, next the substance in the 

 electric field, and then a second nicol. 



If the nicol, as is the case here, is applied to the collimator, then 

 the adjustment of the collimator-lens must be altered, and this adjust- 

 ment is facilitated by the provision of a notch. The inner nicol affords, 

 in contrast with other arrangements, the advantage of greater brightness 

 with moderately small dimensions ; it is also very easily adjustable. 

 Anyone who has once worked with it will scarcely ever abandon it. 



The glass plate also serves for measuring layer-thicknesses and 

 phase-origins by Wernicke and Wiener's methods. Near the collimator- 

 objective O x is a shutter M, which can be regulated up to 0*02 seconds 

 for time and instantaneous exposures. 



The observation tube F and the camera K are easily interchange- 

 able. A glass micrometer divided into hundredths is inserted at the 

 image-plane / of the telescope ; it is illuminated from' a lateral slit. 

 Two diaphragms can also be inserted at / for shutting out desired 

 parts of the spectrum. A Young's prism arrangement produces the 

 dispersion, which can be increased, if desired, by a Cornu double prism. 



The camera K is entirely of metal, and for coincidence photographs 

 a rotatory disc s, with four apertures, is placed in front of the slit. 



Glass Polarizing Prisms.* — Of late years calcite has become not 

 only scarce but difficult to obtain sufficiently large and free from defects. 

 The expense of calcite polarizing prisms is consecpiently considerable. 

 Among other substitutes glass prisms of suitable angle have been sug- 

 gested. H. Schulz quotes a form due to Stotze (fig. 82) in which the 

 angle tv of the prism A B C D E F is so chosen that the ray passing per- 

 pendicularly through A B and D E is totally polarized at incidence on 

 BC, the plane EF being silvered to diminish the weakening of light- 

 intensity due to reflection at its surface. The lateral displacement of 

 the polarized beam is, however, a great disadvantage, and has been an 

 obstacle to the adoption of the method. H. Schulz now proposes a 

 prism of the form shown in fig. 83, by which the path of the emergent 



* Zeit. f. Iustruuieuteuk., xxxi. (l'Jll) pp. 180-2 (2 figs.). 



