200 



Scientific Proceedings^ Royal Duhlin Society, 



light is extinguished (reappearing as the angle is passed), the clamping- 

 screw is then turned, and by means of the vernier the angle is read ofi in 

 degrees and minutes. As the polarizing angle of nearly all substances lies 

 between 48° and 68°, the circle need not be finely graduated for more tlian 

 20°. An enlarged view of scale and vernier is shown in fig. 2. It is, of 

 course, essential that the light incident on the opaque surface be strictly 

 parallel ; and careful adjustment of the collimator must be made beforehand 

 for this purpose. Provision is also made in tlie instrument for two otlier 

 adjustments, namely (1) tlie coincidence of the axis of collimation and the 



zero of the scale, and (2) of the reflecting surface to be tested with the centre 

 of the graduated circle. This latter is accomplished by a small projecting 

 arm with a platinum point, shown in fig. 2. 



Liquids are placed in a little glass capsule on the levelling-table, which 

 is adjusted until the platinum point, indicating the centre of the circle, just 

 touches the liquid surface. Bodies having an irregular, granular, or 

 crystalline surface, if fusible, are melted. This is accomplished by placing 

 them in a small capsule of metal or porcelain, which is heated by a current 

 of steam or an electric current traversing a platinum wire coiled round the 

 capsule. 



