110 A HALF-HOUR WITH 



the pieces of glass into the tube, taking care that 

 the lower edge of the first piece rests on the card- 

 board lodge i. When all the pieces are in posi- 

 tion a similar strip of card must be pasted on the 

 upper part of the opposite side of the tube. The 

 analyzer can of course be constructed the same 

 way. These square tubes can be fitted into cylin- 

 drical ones, and adapted to the fittings of the 

 Microscope. 



Although with this form of polariscope the 

 young student will be able to examine many 

 objects by polarized light, the Nichol prisms are 

 far superior for the purpose, and most of the 

 opticians supply the polarizing apparatus for stu- 

 dents' Microscopes at a moderate cost (from 305. 

 to 35s.). 



Having now described the Micro-polariscope, 

 and the mode of using it, we will proceed to de- 

 scribe a few of those objects to which polarized 

 light may be effectively applied. Matter pos- 

 sessing a crystalline structure as a rule affords 

 the greatest variety of form and colour. The 

 following list of salts, &c., most of which are 

 easily procured, give a brilliant display of colour 

 when polarized : 



Chloride of Barium.* 

 Chlorate of Potash.* 

 Sulphate of Copper,* 



Nickel* 

 Iron.* 



Zinc.* 

 Lime. 



Tartrate of Soda.* 

 Salicine. 



lodo-sulphate of Quinine. 

 Asparagine. 



* Salts marked thus * are most effective when crystaJ- 

 lized in gum or gelatine. 



