PHYSICS. 243 



angle between the optic axes. Parallel light from the elec- 

 tric lamp, after traversing a Nicol of about 2 inches aperture, 

 is rendered divergent by a set of lenses. The crystal under 

 examination is placed in a recess formed by removing a slice 

 from the middle of a spherical lens which is capable of mo- 

 tion in any direction about its centre, while any movement 

 in the vertical plane passing through the axis of the instru- 

 ment can be measured by a scale and vernier; and if, by 

 such a motion, the point on the screen representing the po- 

 sition of one axis, when the two are in the vertical plane, be 

 transferred to that indicating the position initially occupied 

 by the other axis, we have at once a measure of the optic 

 angle of the crystal, since the rotation of two piano-spherical 

 lenses forming an exact sphere has no effect on the direction 

 of the beam. 



Arzruni, in studying the crystalline properties of various 

 organic bodies, has discovered that triphenyl-benzene pos- 

 sesses the property of double refraction to a degree surpass- 

 ing that of any other crystalline body yet known. In sub- 

 stituted compounds he shows also that the introduction of 

 the nitro-group invariably causes a much slighter change in 

 crystallographic properties than when hydrogen is substi- 

 tuted by bromine or by iodine. 



Sarasin has determined with great care the indices of re- 

 fraction of the ordinary and extraordinary rays in quartz in 

 the ultra-violet region of the spectrum. He used a goniom- 

 eter with a collimator and observing telescope, the latter fur- 

 nished with a fluorescent eye-piece. He believes his results 

 accurate to the fourth decimal place, the metals used being 

 cadmium, sodium, zinc, and aluminum. 



Mackenzie has made some experiments in Helmholtz's lab- 

 oratory in Berlin on the relation between electricity and light 

 discovered by Kerr. A glass plate 16.1 centimeters long 

 and 1.2 thick was covered on its opposite sides with tin-foil, 

 the two surfaces being in connection with the secondary wire 

 of a Ruhmkorf coil, and placed between two Nicol prisms. 

 No perceptible increase of brightness was observed on elec- 

 trification, though the high sensibility of the polariscope was 

 fully proved. The conclusion follows that the result ob- 

 served by Kerr is not produced by the electric tension itself, 

 but is possibly an eflect of heat. 



