SCIENTIFIC SUMMARY. 
189 
details, hardly comes within the province of Physics. It is, however, a 
most laborious and exhaustive discussion of a somewhat neglected subject. 
Measurements in Electro-Optics. — Dr. Kerr continues this important 
subject in the Philosophical Magazine. He finds that the dioptric actions of 
dielectrics are generally of one kind — pure double refractions with reference 
to the line of electric force as axis ; but they vary largely in intensity, and 
even in sign, from one dielectric to another. Dielectrics are optically equi- 
valent to uniaxial crystals, and exhibit like variations, both from strong to 
weak, and from positive to negative. Carbon disulphide is of the positive 
class, and in regard to strength holds a place like that of Iceland spar among 
crystals. 
The law is most probably as follows. The intensity of electro-optic 
action of a given dielectric, per unit thickness, varies directly as the square 
of the electric force. 
He describes at length a new form of cell in which a 4-inch plate of 
powerfully refractive and dispersive liquid is enclosed between two conductors 
of brass, the lower resting on its floor, the upper supported by a glass rafter. 
The opposed faces are flat and smooth, parallel, and distant about in. All 
edges and corners are rounded away. The cell is filled with about a pint of 
clean disulphide. Two Nicol prisms are placed in the path of a beam from 
outside, the ray passing between the conductors in a slit four inches long, 
one inch broad, and ^ in. deep, the latter lying vertically as the line of force. 
Wires are led from the lower conductor to earth, and from the upper to the 
prime conductor. On starting the machine, the potential of the upper con- 
ductor rises slowly, and the black space between the conductors is gradually 
illuminated, passing from black, bluish-grey, faint white, to a pure and 
brilliant white. Although the highest potential reached is comparatively 
low, the optical effect is very large, and beyond neutralization by a band- 
compensator of strained glass. As the potential rises, the polariscope gives a 
fine progression of chromatic effects, which descend regularly and continuously 
through a certain range of Newton’s scale. The luminous band passes from 
white to bright straw-colour, deepening to yellow, then through orange to 
deep brown, then to pure and dense red, then to purple and very deep violet, 
then to rich and full brown, then to green. About the last point the process 
generally terminates in spark discharge. The dielectric acts like a plate of 
quartz with optic axis parallel to the lines of force, and increasing in thickness 
rapidly as the potential rises. 
To measure potential he uses a Thomson’s long-range electrometer, and 
a Jamin’s compensator with the axis of its prisms perpendicular to the ray, 
one vertical, the other therefore horizontal. A convex lens gives a di'tinct 
view of the black band and the reference-wires in the compensator. The 
exact process of measurement requires much detail for its description. It 
gave congruous results. In conclusion, the law of squares is stated in several 
forms. The quantity of optical effect varies : 1. Directly as the square of 
the resultant electric force ; 2. directly as the energy of the electric field per 
unit of volume ; 3. directly as the mutual attraction of the two conductors ; 
4. directly as the electric tension of the dielectric : a quantity long ago con- 
ceived clearly by Faraday, and introduced into theory by Clerk Maxwell. 
Their views are strongly confirmed by the new facts. 
