a newly -discovered Salt of Quinine. 169 



centre of the field of the microscope on the stage, and superim- 

 posing upon it a plate of selenite, of such a thickness that it will 

 give a brilliant wine colour, or the complementary green, when 

 examined by the second or analysing tourmaline placed over the 

 eye- piece. But in the experiment now to be described the su- 

 perior plate of tourmaline is not employed. 



In fig. 10 four prismatic crystals of the iodide of disulphate 

 of quinine are supposed to be arranged at various angles of rota- 

 tion, (a) is placed in the position from 0° to 180°, and at right 

 angles to the plane of the tourmaline. This crystal, acting as a 

 tourmaline in the field of the microscope, developes the colour 

 of the selenite stage, and of course appears wine-coloured. 



(b) is across the field at 90° to the former one ; it shows the 

 complementary green. 



The crystals (c) and (d) are across the field at 45° to the plane 

 of the tourmaline ; they are therefore in a position to exert but a 

 minimum of polarizing power : an olive-green tint is produced. 



The force of the argument may not be apparent at first sight, 

 but upon experimenting with the hexagonal plates we are soon 

 convinced of the fact. Here a is at right angles to the tourma- 

 line below the stage, and therefore appears wine-coloured j whilst 

 b is parallel to the polarizing plate, and of course is comple- 

 mentary. 



In the centre of the field the crystals cross at 90°, and 

 therefore polarize. But any lingering doubt we may yet have of 

 the truth of this position is most certainly removed upon pro- 

 ceeding to the following experiment, in which we simply substi- 

 tute for the pink selenite stage a plate of the same substance of 

 a different thickness, one which developes the sky-blue tint in 

 polarized light. 



We now perceive that the crystal which is at right angles to 

 the tourmaline is a beautiful blue, whilst that crystal which is 

 parallel to it is the complementary yellow. The two intermediate 

 crystals are of a slight neutral tint, as they produce but a minor 

 degree of polarizing power at these angles. 



The hexagonal crystals show the same phenomenon, but in a 

 more marked degree. 



The phenomena exhibited by this substance in these experi- 

 ments were so remarkable, and so different from those of any 

 crystals I had previously examined, that I was induced to make 

 a comparative scries of experiments upon some other crystalline 

 compounds, as I felt convinced that the single tourmaline and 

 the selenite stage would become a very delicate test of the power 

 which any substance may possess of polarizing a ray of light. 



I Jpon submitting disulphate of einchonine to this experiment, 

 I found it to possess a decided power of polarizing light. 



