52 



POLARISATION OF LIGHT. 



of the positive one. As the bore of the 

 tube becomes smaller the circle of no 

 double refraction approaches to the axis 

 of the tube. 



5. On the communication of a per- 

 manent doubly refracting structure to 

 Glass. In March 1814, Dr. Brewster 

 found that glass melted and suddenly 

 cooled, such as Prince Rupert's drops, 

 possessed a permanent doubly refracting 

 structure. In December 1814, Dr. See- 

 beck published experiments of a similar 

 kind, with cubes of glass, and Dr. Brew- 

 ster, who had extended his experiments 

 to plates of glass of all forms, analysed 

 the various phenomena which they pro- 

 duced, and published an account of them 

 in the Philosophical Transactions for 

 181G. 



In order to form cylinders, or plates, 

 or spheres, with a permanent doubly re- 

 fracting structure, we have only to bring 

 the glass to a red heat, and cool it ra- 



pidly at its circumference" or its edges. 

 When the cylinders or plates have been 

 thus rapidly cooled, they will produce the 

 very same phenomena which are exhi- 

 bited by plates that have only transiently 

 the doubly refracting structure during 

 the propagation of heat through the mass. 

 The maximum tint developed at the 

 edge of a plate of crown glass 0.44 of an 

 inch thick, was the red of the fifth order 

 of colours. 



6. On the effects produced by sub- 

 dividing or altering the form of Plates 

 and Cylinders of doubly refracting 

 Glass. In doubly refracting crystals, the 

 phenomena which they produce are quite 

 independent of the form of the crystal or 

 portion of a crystal employed. The case, 

 however, is quite different with plates 

 and cylinders of glass, as the effect de- 

 pends, in a great measure, on their ex- 

 ternal shape. If we divide the plate 

 E F C D into two, by a diamond cut, in 

 the direction a b, and separate the two 

 pieces, they will, when exposed to the 

 polarised light, no longer exhibit the 

 fringes which appear injftg. 53, on each 

 side of a b, kutjeach half of the plate will 

 have the same structure as the whole 

 plate had originally, with this difference 

 only, that the tints are all much lower in 

 the scale. This remarkable effect is 

 shewn in fig. 59, in which a b corre- 

 sponds with a b in fig. 53. Each half of 

 the plate has now two lines of no double 



Fig. 59. 



refraction, with one negative structure 

 between them, and two positive structures 

 without them. If the plate had been di- 

 vided in the direction G H, the angular 

 structure at E, as in fig. 53, would have 

 appeared at the four new angles. 



In like manner, if, in the glass cylinder 

 shewn in fig. 57, we cut a notch through 

 it, by means of a file, two lines of no 

 double refraction will appear, a negative 

 structure being interposed between two 

 positive ones. 



The optical figures produced by un- 

 annealed plates and cylinders of glass 

 exhibit very curious variations, by grind- 



ing them into new forms. A cylinder, 

 for example, which gives a circular sys- 

 tem of rings, with a black cross, as in 

 fig. 25, w r ill, if its section is made ellip- 

 tical, give the black cross only when 

 the greater and lesser axis of the ellipse 

 are in the plane of primitive polarisation. 

 When they are inclined 45 to it, the 

 black cross opens in the centre, ex- 

 hibiting the influence of a new axis de- 

 veloped by the elliptical form of the cy- 

 linder. 



7. Description of a Chromatic Ver- 

 nier for subdividing tints. If we take a 

 plate of glass of considerable thickness, 



