'480 Professor Silvamis P. TJiompson [May 17, 



have refined upon the box with partitions, using instead parallel 

 plates of glass mounted in wooden cylinders, whilst for tlie cord 

 swung by hand I am using Professor Schwedoff's device, and am 

 producing the vibrations in this silken cord by means of an electri- 

 cally-driven tuning-fork (Fig. 2). At tlie first nodal point of the 

 stretched cord a pair of parallel glass plates act as a polariser, the 

 cord beyond that jDoint vibrating in the plane thus imposed upon it. I 

 can alter this plane at will by rotating the polariser. This polariser, 

 P, consisting of a pair of glass plates, is mounted in a cylindrical 

 mount, and is provided with an arrow to indicate their direction. 

 If now at any subseii[uent node I introduce a second such device, it 

 will act as an analyser, A. This excellent suggestion is due to M. 

 Mace de Lepinay. In Fig. 2 the jiolariser and analyser are parallel. 

 You see (Fig. 3) how the vibration is extinguished when the positions 

 of analyser and polariser are crossed. Half a degree of error in the 

 position of the analyser produces something less than perfect extinc- 

 tion of the vibrations. Hence it is possible, by this analyser, to 

 determine the plane of the vibrations to the accuracy of half a 

 degree. I should say that the whole of this model has been con- 

 structed by my assistant Mr. Eustace Thomas. 



Now let me show you the optical effect which corresponds to this. 

 Placing a second Nicol prism as analyser in the path of the polarised 

 waves, I turn it to the position where it cuts off the polarised 

 light. The ^^ dark field" so produced by the crossed Nicol prisms 

 corresponds to the motionless cord beyond the crossed analyser of 

 the acoustic apjiaratus. 



Keturning for a moment to two well-known forms of polarising 

 apparatus, viz. the black glass reflector and the Nicol prism, I may be 

 permitted to refer to some recent attempts to improve ujion these 

 devices. 



The Nicol prism, as is well known, consists of a rhomb of 

 Iceland spar cut into two pieces, which are reunited by a film of 

 Canada balsam. As originally devised, 

 Fig. 4. it had oblique end faces (Fig. 4) and a 



comparatively narrow angle (19°) of 

 aperture. These may be noticed in the 

 small example which I here exhibit, which 

 is an original constructed by William 

 Nicol himself. It also has the disad- 

 vantage of giving a field in which the 

 -If directions of the planes of polarisation 



aZJshZscryZZgrZhic'''^^ ^^^ «t^i«% V^^^^^^^ to one another 

 axis. throughout its whole extent. Consequently 



there is never complete extinction of light 

 all over the field at one time. Hartnack and others have 

 attempted to remedy this by giving the prism a different form 

 and using other materials than Canada balsam. I have from time 

 to time made many attempts to improve upon the original construction. 



