38 



It would be found that, when light was received by the bundle of glass 

 plates at the proper angle, which angle varied with different bodies, 

 but was about 56° 35' for glass, a portion of the beam of light was re- 

 fracted and passed through the glass, being absorbed by the blackened 

 surface at the back. If the light that is reflected is examined by another 

 bundle of glass plates or a rhomb of Iceland Spar, known as a Nicol's Prism, 

 the beam no longer behaved as white light, but appeared to possess sides, 

 the light vibrating in a plane corresponding to the parallel surface of the 

 glass. 



By means of a diagram, he showed how the waves of light would foroe 

 their way into the bundle of glass. It would be seen that by using a 

 bundle of thin plates or a Nicol's Prism it was possible to analyse the beam 

 of reflected light. 



In one position the prism allowed the light to pass through, and a bright 

 field was seen, but if the prism was rotated until it was at right angles, no 

 light could pass, and a dark field was the result. This was illustrated by 

 first placing the fingers of one hand parallel with those of the other, and then 

 placing the fingers of one hand at right angles to those, of the other, in whioh 

 latter position they could not, of course, pass in the same plane. 



He next directed attention to a disc of mica, mica being preferable for 

 these experiments because it was far more easily split into large and uni- 

 form films. This disc of mica had a direction in which there was a special 

 strain or tension. An ordinary ray of light, in passing through such a 

 crvstal, was divided into certain vibrations in the direction of this strain 

 and others at right angles to it. 



"When this piece of mica was placed on the table polariscope, with the line 

 of tension at an angle of 45° to the plane of vibration of the polarized raj', 

 the mica would present various colours according to its thickness, because 

 the vibrations in the direction of the particular strain were passing through 

 the crystal with different velocities : the one the ordinary ray, and the other 

 the extraordinary ray, and these differed in their rate of transmission. 



These rays entered the rhomb of Iceland Spar, and were again split up 

 into two by the same law as the original beam, so there were now four sets 

 of vibrations to deal with, two of which were vibrating at right angles to 

 the other two, the extraordinary ray of one set corresponding with the 

 ordinary ray of the other. [This was illustrated and explained by a diagram 

 on the blackboard.] 



Now these four sets of rays, of which two sets were parallel but passing 

 with different velocities, reached the film of Canada balsam which cemented 

 the two halves of the rhomb together ; the vibrations in the plane of the 

 balsam film were reflected to the side and absorbed by the tube in which 

 the rhomb was placed, and the others were transmitted to the eye. 



The result was that two sets of vibrations were wiped out, and two sets 

 remained to be dealt with, which were coincident with each other ; one of 

 these had got out a little in advance of the other. Meeting in opposite 

 phases, the vibrations of a certain length— say red — would clash with the 

 red of the other ; the ether particle would not move at all, but would come 



