POLARISATION OF LIGHT. 145 



" A plate of brass, fig. 86, three inches by one, perforated 

 with a series of holes from about one-sixteenth to one- 



Fig. S3. Red is represented by perpendicular lines ; Green by oblique. 



fourth of an inch in diameter; the size of the smallest 

 should be in accordance with the power of the object-glass, 

 and the separating power of the double refraction. 



" Experiment 1. Place the brass plate so that the smallest 

 hole shall be in the centre of the stage of the instrument ; 

 employ a low power (1^ or 2 inch) object-glass, and adjust 

 the focus as for an ordinary microscopic object; place the 

 double image prism over the eye-piece, and there will 

 appear two distinct images; then, by revolving the prism, 

 these will describe a circle, the circumference of which 

 cuts the centre of the field of view ; the one is called the 

 ordinary, the other the extraordinary ray. By passing the 

 slide along, that the larger orifices may appear in the field, 

 the images will not be completely separated, but will 

 overlap, as represented in the figure. 



" Experiment 2. Screw the Nicol's prism into its place 

 under the stage, still retaining the double image prism 

 over the eye-piece ; then, by examining the object, there 

 will appear in some positions two, but in others only one 

 image; and it will be observed, that at 90 from the latter 

 position this ray will be cut off, and that which was first 

 observed will become visible; at 180, or one-half the 

 circle, an alternate change will take place; at 270, another 

 change; and at 360, or the completion of the circle, the 

 original appearance. 



" Before proceeding to the next experiment, it will be as 

 well to observe the position of the Nicol's prism, which 

 should be adjusted with its angles parallel to the square 

 parts of the stage. In order to secure the greatest 

 brilliancy in the experiment, the proper relative position 

 of the selenite may be determined by noticing the natural 



