tlie Study of tlie Properties of Electric Waves. 67 



The receiver carrying the analyser is also capable of 

 rotation round a horizontal axis by means of a tangent screw. 

 The angular rotation is measured by means of an index fixed 

 to the analyser, and a graduated vertical disk. 



The gratings are made by winding fine copper wire, 

 parallel, round square frames, as used by both Hertz and 

 Lodge. A series of parallel slits cut in a metallic plate serves 

 the purpose very well. Other forms used — the serpentine 

 and the jute polarizers — will be described later on. 



The spark-gap is placed vertical, and the polarizer is 

 adjusted with wires horizontal. The emergent beam is now 

 completely polarized, the vibration taking place in a vertical 

 plane passing through the axis. 



The analyser fitted on to the receiver may be placed in two 

 positions : — 



(1) Parallel position. When both the gratings are hori- 

 zontal. 



(2) Crossed position. When the polarizing grating is 

 horizontal, and the analysing grating vertical. 



In the first position the radiation, being transmitted 

 through both the gratings, falls on the sensitive surface, and 

 the galvanometer responds. The field is then said to be 

 bright. In the second position the radiation is extinguished 

 by the crossed gratings, the galvanometer remains unaffected, 

 and the field is said to be dark. But on interposing certain 

 crystals with their principal planes inclined at 45° to the 

 horizon, the field is partially restored, and the galvanometer 

 spot sweeps across the scale. This is the so-called depolari- 

 zation action of double-refracting substances *. 



Experiments with Wire Gratings. — A wire grating at 45° 

 interposed between the crossed analyser and polarizer partially 

 restores the field, but ordinary wire gauze does not transmit 

 any radiation, the action of one set of wires being neutralized 

 by that of the other set at right angles. 



Double Refraction Produced by Crystals. — The crystals to 

 be examined are mounted on the holder, and properly 

 inclined. Double refraction is shown by all crystals belong- 

 ing to the Khombic, Rhombohedral, Triclinic, and Mono- 

 clinic systems. The effects exhibited by the following are 

 very marked, small pieces even producing depolarization. 



* For a detailed account of experiments on the polarization of the 

 electric raj r , I would refer to my paper, " On the Polarization of the 

 Electric Ray by Double-refracting Crystals," read before the Asiatic 

 Society of Bengal, May 1895, and°two subsequent papers (" On a new 

 Electro-Polariscope " and " On Double-refraction of the Electric Ray by 

 a Strained Dielectric ") published in the ' Electrician,' 27th December, 

 1895. 



