25 8 MOUNTING IN BALSAM [ CH. IX 



2. The object is arranged as desired in the center of a clean slide on the 

 centering card (Fig. 199). 



3. A drop of the mounting medium is put directly upon the object or 

 spread on a cover-glass. 



4. The cover-glass is put on the specimen with fine forceps (Fig. 197), 

 but in no case does one breathe on the cover as when media miscible with 

 water are used. 



5. The cover-glass is pressed down gently. 



6. The slide is labeled. 



7. The preparation is cataloged and safely stored (§ 367). 



\ 355. Example of Mounting in Balsam by Desiccation. — Find a fresh 

 fly, or if in winter, procure a dead one from a window sill or a spider's web. 

 Remove the fly's wings, being especially careful to keep them the dorsal side 

 up. With a camel's hair brush remove any dirt that may be clinging to them. 

 Place a clean slide on the centering card, then with fine forceps put the two 

 wings within one of the guide rings. Leave one dorsal side up, turn the other 

 ventral side up. Spread some Canada balsam on the face of the cover-glass 

 and with the fine forceps place the cover upon the wings (Fig. 197). Prob- 

 ably some air-bubbles will appear in the preparation, but if the slide is put in 

 a warm place these will soon disappear. Label, catalog, etc. 



§ 356. Mounting in Resinous Media by a Series of Displacements. — For 

 examples of this see the procedure in the paraffin and in the collodion methods 

 Ch. X. The first step in the series is Dehydration, that is, the water is dis- 

 placed by some liquid which is miscible both with the water and the next 

 liquid to be used. Strong alcohol (95% or stronger) is usually employed for 

 this. Plenty of it must be used to displace the last trace of water. The tissue 

 may be soaked in a dish of the alcohol, or alcohol from a pipette may be 

 poured upon it. Dehydration usually occurs in the thin objects to be mounted 

 in balsam in 5 to 15 minutes. If a dish of alcohol is used it must not be used 

 too many times, as it loses in strength. 



The second step is clearing. That is, some liquid which is miscible with 

 the alcohol and also with the resinous medium is used. This liquid is highly 

 refractive in most cases, and consequently this step is called clearing and the 

 liquid a clearer. The clearer displaces the alcohol, and renders the object 

 more or less translucent. In case the water was not all removed, a cloudiness 

 will appear in parts or over the whole of the preparation . In this case the prep- 

 aration must be returned to alcohol to complete the dehydration. 



One can tell when a specimen is properly cleared by holding it over some 

 dark object. If it is cleared it can be seen only with difficulty, as but little 

 light is reflected from it. If it is held toward the window, however, it will 

 appear translucent. 



The third and final step is the displacement of the clearer by the resinous 

 mounting medium. 



The specimen is drained of clearer and allowed to stand for a short time 

 till there appears the first sign of dullness from evaporation of the clearer from 



