MOUNTING 15 



forceps, or hold the slip by its edges in an oblique 

 position in the finger and thumb of the left hand, while 

 it is supported by a clean needle held in the right; 

 then, the lower side being wetted with the medium, 

 gradually withdraw the needle and thus gently lower 

 the slip. It would be well at first to practise thus 

 lowering the cover-glass over a drop of water, so as to 

 acquire judgment of the quantity of fluid required, and 

 skill in avoiding the inclosure of air-bubbles. 



6. One great purpose of the above directions is to 

 avoid the presence of bubbles of air in the medium 

 surrounding the object ; their presence is one of the 

 great difficulties of the beginner, who is therefore advised, 

 in his own interest, to follow carefully the directions 

 above given. In some specimens, especially when 

 fresh, air-bubbles will be found entangled in the tissues, 

 or attached to the outside : a good method for avoiding 

 them in mounting fresh material is to moisten with 

 alcohol (weak alcohol will do) for a few seconds before 

 mounting : by this means the surface of the object will 

 be more thoroughly wetted than would otherwise be 

 the case. Obstinate bubbles may be expelled by heating 

 over a spirit-lamp; but as many objects will not stand 

 such rough treatment, a better method is to exhaust 

 them under the receiver of an air-pump. 



7. After an object has been mounted it is often 

 necessary to apply to it certain staining, or niicro- 

 chemical reagents : this may frequently be done, 

 without removing the object from the slide, by 

 irrigation : successive drops of the reagent are placed 

 on the slide, close to one edge of the- cover-slip (special 

 are "being taken that the fluid does not spread to the upper 



