MOUNTING ii 



done either by holding the cover-slip in a pair of clean 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 mount- 

 ing : 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 micro-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 

 care being taken that the fluid does not spread to the upper side 

 of the cover, while a small piece of blotting-paper is pushed 

 up to the opposite edge of the cover-slip, so that, when it 

 comes in contact with the medium in which the object is 

 mounted, it will soak it up : the space thus vacated by the 

 medium is taken by the reagent, and if the latter be supplied 

 in sufficient quantity, a stream of it will pass under the cover- 

 slip and bathe the object. It is obvious that for such a treat- 

 ment to be successful the medium and the reagent must be 



