a newly-discovered Salt of Quinine. 167 



1st. If the two prisms are perfectly parallel in their long dia- 

 meters, the ray of light will pass through unaltered. (Figs.3and4.) 

 2nd. When they cross each other at a right angle, the small 

 square spot where they cross will be as black as midnight. 

 The two rays are both obstructed ; that is, about half the inci- 

 dent ray of ordinary light is stopped or absorbed by the first or 

 lowest crystal, the other half transmitted by it in a polarized 

 state ; this impinging upon the superior crystal is stopped by it 

 effectually. 



3rd. When the two crystals intersect each other at an angle 

 of 45°, polarization also occurs, but not to the same extent ; the 

 spot where they are superposed is decidedly darkened. 



4th. There is a perceptible polarizing effect produced at an 

 angle of 30° ; below this there does not appear to be any effect 

 on the transmitted light. 



5th. Similar effects are equally well observed in the superpo- 

 sition of the hexagonal plates and other forms. (See figs. 4 and 5 .) 

 (C.) When three crystals are examined. in a superimposed 

 condition, two being crossed at right angles, and therefore dark, 

 and a third introduced between them, the phenomena of depo- 

 larization are produced : the interposed crystal permits the light 

 to pass through, and at the same time communicates to it the 

 order of colour, equivalent to its thickness, in the same manner 

 as a plate of selenite would do if interposed between two tour- 

 malines at right angles to each other. 



The angle of depolarization appears to be 45° to the plane of 

 either polarizing or analysing crystal; but the phenomenon will 

 take place in a minor degree at other angles. (Vide fig. 5.) 



Similar phenomena are produced by tbe hexagonal plates and 

 other crystalline forms. (Vide fig. 5.) 



(D.) The action of a single tourmaline or Nicol's prism is 

 very marked, and proves beyond a doubt that these crystals 

 possess both the polarizing and the depolarizing powers. 



In the first place, upon examining two of these crystals placed 

 at right angles the one to the other, with a single tourmaline or 

 NicoPs prism, one crystal is perfectly black and obstructs all the 

 light, the other is as transparent as ever. Upon more closely 

 analysing this experiment, it will be found that the crystal whose 

 length crosses the plane of the tourmaline at right angles is the 

 dark one, whilst that one whose long diameter is parallel to the 

 plane of the tourmaline is transparent. (Fig. 6.) 



Upon rotating the tourmaline 90°, or one quarter of a circle, 

 the crystal which was before transparent becomes dark, and that 

 which was dark now becomes transparent. (Figs. and 7.) 

 Their polarizing power may now he considered to be established. 

 It remains to prove their depolarizing power with equal cer- 

 tainty. 



