706 



Professor Andrew Gray 



[April 29, 



is readied in which the light is again freely transmitted, and so on, the 

 light being alternately stopped and transmitted by the second prisms in 

 successive positions 90° apart. 



The mode of passage of the wave by the Nicols when their planes 

 are parallel, and its stoppage when the planes are crossed, are illus- 

 trated by this diagram (Fig. 3) of a vibrating cord and two slits. 

 When the slits are parallel, the vibration which is passed by one is 

 passed by the other ; when they are crossed, a vibration passed by one 

 is stopped by the other. 



Two planes of symmetry of the prisms parallel to the ray, and 

 called their principal planes, are parallel to one another when the 

 light passes through both, and are perpendicular to one another 

 when the light passed by the first is sto23ped by the second. We shall 

 call the first prism the polarising prism, or the ^oZamer, from its effect 

 in producing plane polarised light ; the other, the analyser. The 

 stoppage of the light in the two positions 180° apart of the second 

 prism, and its passage in the two intermediate positions, show that the 

 light passed by the first prism is plane polarised. 



Fig. 4. 



Now a beam of plane polarised light is passed through the per- 

 forated pole-pieces of this large electro-magnet (Fig. 4), so that the 

 beam travels between the pole-faces along the direction which the lines 

 of force there would have if the magnet were excited by a current. The 

 arrangement of the apparatus is as shown in the diagram. The light 

 is polarised by the prism P, passes through the magnetic field, and 

 then through the analysing prism A, to the screen. As you see, when 

 the second prism is turned round the ray the light on the screen alter- 

 nately shines out and is extinguished, and you can see also that the 

 angle between the positions of free passage and extinction is 90°. 



I now place in the path of the beam this bar of a very remarkable 

 kind of glass, some of the j)roperties of which were investigated by 

 Faraday. It is a very dense kind of lead glass, which may be 

 described as a silicated borate of lead ; that is, it contains silica, boric 

 acid and lead oxide. The beam is not disturbed although the light 

 passes through the glass from end to end. I now adjust the analysing 

 prism to very nearly complete extinction, and then excite the magnet. 



