44^ Iceland Spar and the Polariscope. [Sess, 



The actual processes of the formation of crystals may be 

 viewed under the microscope with facility, all that is neces- 

 sary being to lay on a slip of glass, previously warmed, a 

 saturated solution of the substance about to be examined, and 

 to incline the stage in a slight degree, so that the drop shall 

 be thicker at its lower edge. The crystallisation will speedily 

 begin at its upper edge, where there is the least liquid, and 

 the evaporation will gradually extend downwards. If it should 

 proceed too slowly, or should cease altogether, while still some 

 of the liquid remains, the part already dissolved should be re- 

 dissolved, after which the process will recommence with increased 

 rapidity. It becomes very much more interesting, however, when 

 the crystals, as they form, are made to stand out bright upon a 

 dark ground by use of the spot lens, the paraboloid, or any 

 other form of dark-ground illumination, but it is still more 

 beautiful with the aid of the polarising apparatus. Besides 

 crystals there are other substances which may be much better 

 observed by the use of the polariscope, such as cuticles of 

 hairs and scales from leaves, fibres of cotton and flax, starch 

 grains, and longitudinal sections of wood, &c. From the 

 animal kingdom we have fibres of sponges, cuttlefish bone, 

 scales of fishes, and sections of hair, quills, shells, skin, teeth, 

 tendons, &c. From a medical point of view the polariscope 

 has proved of immense value in its application to the discover- 

 ing of the salts of alkaloids, quinine, &c., in the urine of 

 patients. It is possible, I believe, by certain processes, to 

 discover, for example, the presence of quinine in quantities so 

 exceedingly minute that all other methods would fail to show 

 its existence. What is called a selenite is very often used in 

 conjunction with the polariscope, which exhibits under ordin- 

 ary circumstances the red ray in one position and the green 

 in the other. Selenite is the native crystallised hydrated 

 sulphate of lime. A very beautiful variety called satin 

 gypsum is found in Derbyshire, very large crystals being also 

 found at Montmartre, near Paris. The thickness of the film 

 of selenite determines the particular tint, so that, if we use a 

 film of irregular thickness, different colours are presented by 

 the different thicknesses. It is usually slit into thin laminae 

 parallel to the large lateral faces, the film having a thickness 

 of 1-2 0th to 1-6 0th of an inch. By a combination of micro- 



