the Study of the Properties of Electric Waves. 63 



Laws of Reflexion. 



Plane Mirror. — A parallel beam is used. The spectro- 

 meter-circle is adjusted with the zero division opposite to the 

 radiator. The platform index is turned to zero, and a plane 

 reflector placed on a previously marked diameter at right 

 angles to the index. The receiver is placed, say, at 60°. The 

 platform carrying the mirror is slowly rotated (electric 

 radiation being at the same time produced by interrupting 

 the key), till the receiver suddenly responds. It will now 

 be found that the platform index points to 30°, midway 

 between the radiator and the receiver. 



Curved Mirror. — A cylindrical metallic mirror, with a 

 radius of 25 cm., is placed on the platform, with its principal 

 axis coinciding with the platform index. When the radiator 

 is placed at a distance of 25 cm. from the mirror, the source 

 of radiation would be at the axis of the cylinder. The reflected 

 image will now be formed at an equal distance. The receiver 

 mounted on the radial arm (at a distance of 25 cm. from the 

 centre) is placed at a given angle ; the platform is rotated 

 till the receiver responds. The index will now be found 

 to bisect the angle included between the radiator and the 

 receiver. 



Refraction . 



Deviation of the Electric Ray by a Prism. — An isosceles 

 right-angled prism is made of sulphur or ebonite. Parallel 

 beam is used. For showing deviation by refraction one of 

 the acute angles is interposed on the path of the beam. 



Total Reflexion. — An interesting experiment on total 

 reflexion is shown in the following way : — The receiver 

 is placed opposite to the radiator, and the prism interposed 

 with one of its equal faces at right angles to the direction of 

 the ray. The receiver will remain unaffected. The critical 

 angle of ebonite being considerably less than 45°, the rays 

 undergo total reflexion. On turning the receiver through 

 90° it responds to the totally reflected ray. 



Opacity due to Multiple Refraction and Reflexion. — An 

 experiment analogous to the opacity of powdered glass to 

 light is shown by filling a long trough with irregular-shaped 

 pieces of pitch, and interposing it between the radiator and 

 the receiver. The electric ray is unable to pass throuo-h the 

 heterogeneous media, owing to the multiplicity of refractions 

 and reflexions, and the receiver remains unaffected. But on 

 restoring partial homogeneity by pouring in kerosene, which 

 has about the same refractive index as pitch, the radiation 

 is easily transmitted. 



