174 The Microscope. 



impeded. This is still true when one of the prisms is rotated 

 180° around from the first position. But at 90° between, on 

 either side, total darkness results, since then we have to imagine 

 two sets of parallel wires, at right angles to each other, so that 

 all other vibration is stopped. 



In themselves, these phenomena are not very interesting. But 

 if different transparent objects, as scales of mica, or other thin 

 sections ^f transparent minerals, thin sections of plant or ani- 

 mal tissues, starch grains, wool or cotton fibres, drops of chemi- 

 cal salts evaporated on glass slips, old lard, and similar 

 srbstances be interposed between the polarizer and analyzer, 

 iiid one of the prisms be then revolved, a display and shifting 

 ■of colors is then observed which in exquisiteness and dazzling 

 beauty surpasses anything ever seen with the naked eye. For 

 most objects, especially the preparations of chemical salts, a f 

 inch objective is satisfactory. While for such as vegetable thin 

 sections a i objective is desirable. Among the latter, the cross 

 section of a Nuphar stem makes a fine object. Some of the most 

 satisfactory preparations of salts are salicin, santonin, tartaric 

 acid, cream of tartar, cadmium bromide and sulphate, potassium 

 ferrocyanide and citrate and rock candy. Salicin requires to be 

 crystallized from a hot solution. Deliquescent salts, as calcium 

 chloride and nitrate, should be warmed on the side, when the 

 process of crystallization can be well observed. Such remain as 

 liquid drops on the slide for a long time, and can be repeatedly 

 heated. Those salts which effloresce make preparations of only 

 short duration. Some salts, especially of ammonium, evaporate 

 sooner or later. But many of these preparations keep well for 

 months, even without cover glass, if protected from moisture and 

 dust. It should be added that some salts, as the chlorides of 

 potassium, sodium and ammonium, give no play of color. Also 

 the crystals of colored salts, as potassium permanganate, copper 

 sulphate, generally do not change color. Of the natural miner- 

 als, thin sections of gneiss, granite, marble, gypsum, hornblende, 

 and agate, are among the most brilliant polariscope objects. But 

 here it must be remembered that there is a maximum and mini- 

 mum'thickness, varying with different minerals, outside of which 

 this play of colors cannot be produced. Thus sulphate of lime 

 may have a minimum thickness of .425 mm., and a maximum 

 of 1.27 mm. Mica must be at most .85 mm. thick; crystalline 



