ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



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are adjusted until the reflecting surface is level, and at the centre of 

 the graduated circle round which travel the telescope and collimator. 

 A small but brilliant source of light is employed, such as a Nernst or a 

 lo-volt electric glow-lamp ; a small lens throws a brilliant image of the 

 light on to the adjustable slit of the collimator. This latter contains a 

 lens in a draw-tube, so that a parallel beam falls on the opaque reflecting 

 surface : and a sharp image of the slit is obtained by the lens in the 

 telescope, which also contains a small Xicol's prism. Upon turning the 

 rack-work handle the source of light and collimator move together, and 

 through the same angle as the telescope. The observer now turns the 

 polarising plane of the Xicol at right angles to the plane of the reflected 



Fig. 8. 



polarised ray, and watches the gradual extinction of the light as the 

 polarising angle is approached. At the angle of maximum polarisation 

 the 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 off in degrees and minutes. As the polarising angle 

 of nearly all substances lies between 48° and (!S°, the circle need not 

 be finely graduated for more than 20°. An enlarged view of scale and 

 vernier is shown in fig. 8. It is, of course, essential that the light 

 incident on the opaque surface be strictly parallel : and careful adjust- 

 ment of the collimator must be made beforehand for this purpose. 

 Provision is also made in the instrument for two other adjustments, 

 namely (1) the coincidence of the axis of collimation and the zero of 



