1 70 Dr. Herapath on the Optical Properties of 



This substance crystallizes as a tuft of minute radiating 

 aeicuhe, sometimes arranged in a perfect circle. (Pig. 11.) Upon 

 placing such a tuft above the red selenite stage, having a tour- 

 maline beneath it, one-half the circle appears red, the other half 

 green. But there are four segments to the circle; one quarter 

 red, one green, one red, and the fourth green. 



Now all those prisms which are arranged at right angles to 

 the plane of the tourmaline are red ; all those parallel to it are 

 the complementary green ; but as the power exists in a minor 

 degree on each side of this line through an arc of 4-5°, of course 

 we get the whole half-circle so coloured, but in two quarter-seg- 

 ments placed apex to apex. 



The same phenomenon is also to be found with the blue sele- 

 nite stage, but it requires a better light of illumination to dis- 

 cover it : the segments are respectively blue and yellow alter- 

 nately. 



Upon examining pure cinchonine in a crystalline state as de- 

 posited from its hot alcoholic solution, the evidence of the same 

 power exhibits itself. 



The oxalurate of magnesia (discovered in the urine of a patient 

 afflicted with the oxalic acid diathesis, after having administered 

 the bicarbonate of magnesia for some time) possesses the same 

 power to a considerable extent. The colours are shown in these 

 dumb-bell crystals with tolerable splendour. 



Taurine possesses this power in a very slight degree only. 



Some radiating crystals of carbonate of lime or magnesia found 

 between the tegumentary layers of the shrimp are also capable 

 of polarizing, or rather analysing the ray to a slight degree. 



The nitrate of urea, the oxalate of urea, nitrate of potassa, and 

 nitrate of soda (rhombic), possess this power in great splendour. 



There is very little doubt that more time spent in the investi- 

 gation of this phenomenon would considerably enlarge the cata- 

 logue of those substances which possess the faculty of polarizing 

 light ; but none of those enumerated possess it to the extent of 

 the iodide of the disulphate of quinine. 



The disulphate of quinine does not exert the slightest influence 

 upon the ray of light under these circumstances. 



(G.) To return from our digression to the point from which 

 we started, our next mode of examining the properties of the new 

 crystals will be by the two tourmalines and by the selenite stage. 



Fig. 12 shows four crystals arranged at various angles as before. 



It wdl be understood that in this experiment the two tour- 

 malines are arranged at right angles ; one being on the stage, 

 the other upon the eye-piece of the microscope ; the plate of 

 selenite superimposed on the inferior tourmaline. In fact, it is 

 the same as the last arrangement, but with the addition of the 



