464 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
which possesses almost the same refractive index as the quartz, and 
the preparation is brought under the Microscope between two nieols 
approximately crossed, but with directions of vibration oblique to the 
cross-wires. The long edge of the crystal is then adjusted parallel to 
the cross-wire, and the polariser is turned until the crystal cannot be 
distinguished from the rest of the field ; the analyser is then turned to 
the position of darkness. If during this rotation the crystal, in any 
position becomes again visible, the polariser must be turned until the 
crystal remains perfectly invisible during the rotation of the analyser 
through 180°. The direction of vibration of the polariser will then be 
parallel to the direction of vibration in the crystal, i.e. to the cross-wire. 
The analyser can be adjusted in a similar way. 
Optical Rule.* — Mr. E. M. Nelson describes a useful optical rule. 
The rule, which is made of box and is 20 in. long and square in 
section, has on one face a scale of inches and tenths, and on the other 
side centimetres and millimetres. On one of the sides, at right angles 
to these, is a scale of dioptrics marked D, and on the opposite side a 
new scale of powers marked P. The author gives several examples 
illustrating the use of the rule. 
Pulfrich Refractometer.'f— This instrument in its new form as con- 
structed by Zeiss is shown in fig. 76. It is intended as a universal 
apparatus for refractrometric and spectrometric investigations. 
In principle the apparatus depends on the use of a rectangular prism 
of strongly refracting glass, of which the horizontal face is brought in 
contact with the object to be examined, while through the second vertical 
Fig. 76. 
Fig. 77. 
face the line of demarcation of the light falling at grazing incidence on 
the object is observed. From the angle i under which the limiting ray 
leaves the vertical face of the prism (see figs. 76 and 77), and from the 
known index (N) of the prism, the refractive index n of the substance 
examined is given by the formula n = VN- — sin 2 i. A glass tube (fig. 76) 
cemented on to the prism serves for the reception of liquids. Solid 
bodies (fig. 77) are provided with two faces I and II at right angles to 
* Journ. Quekett Micr. Soc., vi. (1896) pp. 208-9. ; 
t Jena, 1895, 8vo, 8 pp. and 4 figs. 
