USE OF REAGENTS 283 



rotation through 360° the illumination of the field changes from 

 l)rightness to blackness and back to brightness again. Turn the 

 analyzer so that the field is at its brightest and place on the stage 

 some potato starch mounted in a drop of water or dilute glycerine; 

 these will then appear when looking through the eyepiece as 

 though no polarizing apparatus were employed. Now turn the 

 analyzer through 180° or until the field is black and each starch 

 grain will be seen to be traversed with a bright cross. It will be 

 found that when thin sections of plant tissues containing crystals 

 of calcium oxalate are treated in the same way some portions of 

 them shine brightly out in the dark field, and the same thing is 

 true of many sclerenchyma cells and fibers. It will be seen from 

 this that the use of the polarizing apparatus may be of great 

 service in identifying the parts of powdered foods and drugs 

 where the fragmentary condition of tissues, cells, and cell 

 contents increases the difficulty of finding out what the differ- 

 ent parts are. 



The Use of Reagents on Microscopic Preparations. — 

 When an object has been mounted in a drop of water under a 

 coverglass the water can be replaced with other fluid reagents 

 without removing the coverglass or removing the preparation 

 from its position under the objective. Suppose we wish to treat 

 with a solution of iodine, starch from the potato that we have 

 already examined in water: put a small drop of the iodine upon 

 the glass slip close to but not touching the coverglass, and then 

 with a dissecting needle or broomstraw drag the drop into 

 contact with the coverglass, when it will diffuse under and give 

 to the starch the characteristic blue color which iodine is known 

 to impart to it. The drop of reagent is not put at first in contact 

 with the coverglass because there would then be danger of its 

 running over the top of the latter, wetting the front lens of the 

 objective and preventing a good image. If this accident should 

 ever happen in the use of a reagent there is nothing to do but to 

 remove and wash the coverglass, and wash and wipe dry the 

 objective lens. Sometimes it is desirable to hasten the replace- 

 ment of the water under the co\erglass by the reagent, and to do 



