BLACKWELL. — DISPERSION IN ELECTRIC DOUBLE REFRACTION. 653 



use of a camera and orthochromatic plates in place of the usual observing 

 telescope permanent records may be obtained. This is an advantage, 

 because measurements of the negatives can be made at leisure more accu- 

 rately than visual observations under the pressure of experiment. 



Apparatus. 



Accurate measurement of double refraction in a strong electric field 

 requires above all a source of constant electromotive force. The Jeffer- 

 son Physical Laboratory is especially well fitted for such work in having 

 a storage battery of twenty thousand cells. 



Electrodes. — The success of the method depends upon the constancy 

 and uniformity of the electric field. Plane, parallel electrodes are neces- 



Figure 2. Section and end view, actual size, of silvered glass bar used as 

 electrode, showing method of connecting to silver film. The film extends from the 

 plane face around the corners and up the sides higher than the wire. 



sary to secure uniformity, and the distance between them must be every- 

 where the same, and accurately known. In order to reduce the voltage 

 required to produce a given strength of field, the distance, " a, " between 

 the bars was made only 0.1607 cm. Since the field-strength is the 

 difference of potential divided by this distance, and enters the expression 

 for the double refraction as the square, the percentage error of " a " is 

 doubled in the result. To keep the error in distance between electrodes 

 negligible, the electrodes were made of silver deposited in a thin bright 

 film on two heavy glass bars whose faces were prepared at the optical 

 works of the Alvan Clark & Sons Corporation. The Newton's fringes 

 formed when a plane test-plate was placed anywhere on these surfaces 

 before they were silvered showed them to be far more nearly plane than 



