CH. VII] MOUNTING AND LABELING. 153 



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. 123). Probably 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., (§ 291- 



295)- 



§ 243. Mounting in Resinous Media by a Series of Displace- 

 ments. — For examples of this see the procedure in the paraffin and in 

 the collodion methods (§ 265, 284). The first step in the series is De- 

 hydration, that is, the water is displaced by some liquid which is misci- 

 ble 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 alco- 

 hol, 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 preparation 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 011I3' 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 resin- 

 ous 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 the surface. Then a drop of the resinous medium is put 

 on the object, and finally a cover-glass is placed over it, or a drop of 

 the mounting medium is spread on the cover and it is then put on the 

 object. 



