a neivly -discovered Salt of Quinine. 171 



superior tourmaline. We will first employ the pink selenite 

 stage. 



The prism (a) is at right angles to the inferior tourmaline and 

 parallel to the superior ; it developes the red colour of the stage. 



The prism (b), being parallel with the inferior tourmaline, is 

 at right angles to the superior tourmaline ; it consequently ob- 

 structs the whole of the light. 



But the crystals c and d are at 45° to either tourmaline, and 

 therefore at that angle which is most favourable for showing the 

 phenomena of depolarization. They are coloured green and 

 yellow respectively, as they now add the influence of their own 

 thickness to that of the selenite stage. 



The experiment being varied by employing the blue selenite 

 plate, all the other arrangements being as before, of course the 

 field will be blue. 



The prism (a) becomes blue from the same cause, the superior 

 tourmaline having no influence upon it. 



The crystal (b) is dark, as before, the superior tourmaline 

 obstructing the beam polarized by it. 



The crystals c and d are now violet and orange respectively, 

 being complementary in colour the one to the other. They act 

 as depolarizing crystals to the light polarized by the inferior 

 tourmaline, and analysed by the superior tourmaline, and add 

 their thickness to the selenite stage ; in this position they exert 

 the same influence upon polarized light that any other crystalline 

 substance belonging to the rhombic prismatic series would do 

 under similar circumstances. 



Upon revolving the superior tourmaline, the whole appear- 

 ance changes ; the field passes to green with one stage and yellow 

 with the other. The crystals pass through various changes in 

 colour and appearance, each in its turn becoming a polarizer in 

 action with the superior tourmaline. 



(H.) It has already been stated that the author has succeeded 

 in adapting one of these artificial tourmalines to the stage of his 

 microscope ; it is sufficiently large to give an uniform tint to the 

 whole field, and covers a surface of an eighth of an inch in dia- 

 meter. This crystal will bear magnifying to any extent, and he 

 has been enabled to illuminate a field of eleven inches in dia- 

 meter with light polarized by its means. 



It is at once evident that such a crystalline plate would be 

 far too small to be serviceable as the analysing plate above the 

 eye-piece ; ;t crystal of at least half an inch in diameter woidd be 

 ary t'>>v this purpose. There is frequently found in the 

 toother-liquid a crystal ntfficientry large t<> be so used, were it 

 passible t« transfer it safely from the fluid to a plate of glass in 

 order to mount, it : the difficulty consists in the extreme fragility 



