85 



The following experiments, if carefully performed, will illustrate 

 the most striking phenomena of double refraction, and form an useful 

 introduction to the practical application of this principle. 

 The apparatus necessary is — 



A Nicol's prism to be adapted under the stage ; 



A double refractor adapted to the eye-pieces ; 



A film of selenite, of uniform thickness, adapted according to its 



crystalline axis ; and 

 A plate of brass, 3 inches by 1, perforated with a series of holes, 

 from about the -^th to £th of an inch in diameter : the di- 

 ameter of the smallest hole should be regulated according to 

 the power of the object-glass and the separating property of 

 the double refractor. 

 Exp. 1. Place the piece of brass so as for the smallest hole to be 

 in the centre of the stage of the instrument, employing a low power 

 (about 1^ inch) object-glass, and adjust the focus as for an ordinary 

 microscopic object; place the doubly-refracting crystal over the eye- 

 piece, and there will appear two distinct images ; then by revolving 

 the eye-piece the images will describe a circle, the circumference of 

 which cuts the centre of the field of view ; the one is called the ordi- 

 nary, and the other the extraordinary ray. By passing the slide 

 along, so as for the larger orifices to appear in the field, the images 

 will not be completely separated, but will overlap as in fig. 4. 



Exp. 2. Screw the Nicol's prism into its place under the stage, 

 still retaining the double refractor over the eye-piece ; then by ex- 

 amining 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 ob- 

 served will become visible ; at 180°, or one half of the circle, an 

 alternate change will take place ; at 270° another change ; and at 

 360°, or the completion of the circle, the original appearance (see 

 fig. 6). 



Before proceeding to the next experiment, it will be as well to ob- 

 serve the position of the Nicol's prism, which should be adjusted with 

 its angles parallel to the square parts of the stage (see fig. 10), in 

 order to secure the greatest brilliancy in the experiment : the proper 

 relative position of the selenite may be determined by noticing the 

 natural fractures or flaws in the film, which will be observed to run 

 parallel to one another : these flaws should be adjusted at about 45° 

 from the square parts of the stage (see fig. 11) to obtain the greatest 

 amount of depolarization. 



TRANS. MIC. SOC. VOL. II. H 



