HEEAPATHITE. 143 



neutral axes, these films have the property of double 

 refraction. These directions are known as the depolarising 

 axes. 



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 beau- 

 tiful 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." The best 

 film for the microscope is that which gives blue, and its 

 complementary colour yellow. Mr. Darker has constructed 

 a very neat stage of brass for this purpose, producing a 

 mixture of all the colours by superimposing 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. When objects are 

 thus exhibited, we must bear in mind that all the negative 

 tints, as we term them, are diminished, and all the 

 positive ones increased ; the effect of this plate is to mask 

 the true character of the phenomena. Polarised structures 

 should therefore never be drawn and coloured under such 

 conditions. 



Dr. Herapath, of Bristol, 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 metallio 

 lustre ; they appear like portions of the ely trss of cantha- 

 rides, and are also very similar to murexide in appearance. 

 When examined by transmitted light, they scarcely possess 



