56 



POLARISATION OF LIGHT. 



duced by thin plates of crystals were 

 modified by compression and dilatation, 

 according to laws, of which the reader 

 will find a full account in the Phil. 

 Trans, for 1816, and the Edin. Trans. 

 vol. viii. The principles thus developed 

 have been applied by Dr. Brewster to 

 the construction of a chromatic dyna- 

 mometer, a chromatic hygrometer, and 

 a chromatic thermometer, different 

 from the one formerly mentioned. In 

 these cases, either direct force, or force 

 arising from expansion, is to be mea- 

 sured; and in the instruments under 

 consideration the force is measured and 

 indicated by the tints developed in, plate 

 of glass bent by the force to be measured. 

 The perfect elasticity of glass gives it a 

 vast superiority over steel, as it will in- 

 variably return to its original state after 

 being bent ; and as the tints of polarised 

 light have a precise numerical value, and 

 may be subdivided and read off by the 

 chromatic vernier described in p. 53, the 

 results of such instruments will merit 

 great confidence. 



It is not difficult to form an idea of 

 the manner in which heat and pressure 

 produce that mechanical change in the 

 condition of the glass which gives rise 

 to the very singular phenomena above 

 described. When Dr. Brewster first 

 saw the extraordinary phenomena of a 

 doubly refracting structure produced 

 at parts of the glass where the heat 

 had not arrived, he was struck with 

 its analogy to the phenomenon in mag- 

 netism in which the production of south 

 polar magnetism at one end of a needle 

 instantly creates north polar magnetism 

 at the other end, and he was disposed to 

 believe that the phenomena might be 

 owing, as in magnetism, to the action of 

 a fluid. This opinion, however, was 

 soon abandoned, after he was better ac- 

 quainted with the influence of compres- 

 sion and dilatation in developing the 

 doubly refracting structure, and he gave 

 the following explanation of the pheno- 

 mena. Let CDFE, Jig. 67, be an 



Fig. 67. 



elastic transparent substance like 

 caoutchouc or isinglass in a particular 

 state of induration, and let it be dilated or 

 drawn out in the direction A B by forces 



applied at A and B : the obvious effect of 

 this is to shorten the sides C D, E F; or, 

 what is the same thing, to produce a 

 compression along these lines. There 

 will consequently be neutral lines m n, 

 op separating the dilated from the com- 

 pressed portions, and if the plate is ex- 

 posed to polarised light, we shall have 

 a positive structure A B between two 

 negative structures CD, E F. If, on 

 the other hand, we compress C D and 

 E F, the effect of this will be to cause a 

 protrusion at A and B, or to dilate the 

 substance in the direction AB. If we 

 apply the compressing force only to E F 

 in a hard elastic substance like glass, 

 this will necessarily produce a protrusion 

 at A and B, or a dilatation in that direc- 

 tion, and this dilatation will produce a 

 compression at CD, though no force is 

 applied there. Hence it is obvious why 

 tints appear at C D when the edge E F 

 of a piece of glass CDFE is laid upon 

 a piece of hot iron. 



In the experiment shown in fg. 65, 

 with the bent plate of glass, the particles 

 of the glass are compressed on the con- 

 cave side C D and in the direction C D, 

 whereas, on the convex side, A B, they 

 are dilated in the same direction as that 

 of A B : hence there must be some neu- 

 tral line M N in which there is a line of 

 particles neither compressed nor dilated. 



From these principles, it is easy to 

 understand how the structure of unan- 

 nealed glass, as indicated by the optical 

 figure which it produces, changes by di- 

 viding it in two, or by altering its form. 

 The structure of the optical figure de- 

 pends on the form of the glass as a whole, 

 and it is easy to conceive how portions 

 kept in a state'of compression by dilating 

 forces in another part of the glass, should 

 lose their state of compression by the re- 

 moval of the dilating forces which occa- 

 sioned it, or should even be thrown into 

 a state of dilatation by the influence of 

 the remaining compressing forces. 



Mr. Herschel has, in his Treatise on 

 Light, given an analogous view of the 

 subject as a theoiy of the phenomena. 

 We regard the views given above as the 

 result of direct experiment. 



CHAPTER XVI. 



Structure of composite Doubly Refract- 

 ing Crystals Brazilian Topaz 

 Sulphate of Potash Remarkable for- 

 mation of tesselated Apophyllite. 



IN examining the phenomena exhibited 

 by doubly refracting crystals, the reader 



