POLARISED LIGHT. Ill 



directions, however, which form respectively angles of 45 with the 

 neutral axis, these films have the property of double refraction. These 

 directions are known as the depolarising doubly refracting axis. 



The thickness of the film of selenite determines the particular tint. 

 If, therefore, we use a film of irregular thickness, different colours are 

 presented by the different thicknesses. These facts admit of very 

 curious and beautiful illustration, when used under the object placed 

 on the stage of the microscope. The films employed should be mounted 

 between two glasses for protection. Some persons employ a large film 

 mounted in this way between plates of glass, with a raised edge, to act 

 as a stage for supporting the object, it is then called the " selenite 

 stage." Mr. Darker has constructed a very neat stage of brass for this 

 purpose, and that of producing a mixture of all the colours by super- 

 imposing three films, one on the other ; by a slight variation in their 

 positions, produced by means of an endless-screw motion, all the colours 

 of the spectrum are shown. 



Dr. Herapath, of Bristol, has recently described a salt of quinine, 

 which is remarkable for its polarising properties. The salt was first 

 accidentally observed by Mr. Phelps, a pupil of Dr. Herapath' s, in a 

 bottle which contained a solution of disulphate of quinine : the salt is 

 formed by dissolving disulphate of quinine in concentrated acetic acid, 

 then warming the solution, and dropping into it carefully, and by small 

 quantities at a time, a spirituous solution of iodine. On placing this 

 mixture aside for some hours, brilliant plates of the new salt will be 

 formed. The crystals of this salt, when examined by reflected light, 

 have a brilliant emerald-green colour, with almost a metallic lustre ; 

 they appear like portions of the elytrse of cantharides, and are also very 

 similar to murexide in appearance. When examined by transmitted 

 light, they scarcely possess any colour, there is only a slightly olive- 

 green tinge ; but if two crystals, crossing at right angles, be examined, 

 the spot where they intersect appears perfectly black, even if the crys- 

 tals are not one five-hundreth of an inch in thickness. If the light be 

 in the slightest degree polarised as by reflection from a cloud, or by 

 the blue sky, or from the glass surface of the mirror of the microscope 

 placed at the polarising angle 56 45' these little prisms immediately 

 assume complementary colours : one appears green, and the other pink, 

 and the part at which they cross is a chocolate or deep chestnut-brown, 

 instead of black. As the result of a series of very elaborate experi- 

 ments, Dr. Herapath finds that this salt possesses the properties of 

 tourmaline in a very exalted degree, as well as of a plate of selenite ; 

 so that it combines the properties of polarising a ray and of depolarising 



