PREPARATION AND TREATMENT OF SPECIMENS. 279 



the two-edged knife for cutting thin lamellie of hard objects of 

 considerable thickness, and so on. When the case arises that one 

 or other instrument can be used with advantage, the most suit- 

 able shape and size are easily selected, and choice if choice there 

 be may be made accordingly. 



If the prepared sections are opaque in consequence of their 

 containing air, we must endeavour to remove it. It may often 

 suffice to leave the section some considerable time in the liquid, 

 provided this is capable of absorbing more air ; boiled water, for 

 instance, is a perfectly safe medium, as also alcohol, according to 

 Schacht. In other cases a slight heating of the preparation over 

 a spirit-lamp answers the purpose. The most effective and con- 

 venient device, however, is the air-pump, a few strokes of which 

 are sufficient to render the objects placed under the receiver quite 

 free from air. If a large air-pump is too expensive, a smaller one 

 in the form of a syringe can easily be made. Into a tube, of 

 about one inch in diameter, closed at the bottom, is fitted a piston 

 provided with a valve, which allows the air to escape on the 

 downward stroke, but checks its entrance on the upward stroke. 

 With this contrivance the object under investigation must be cut 

 in small pieces and placed in the tube with a little water before 

 its preparation, and must afterwards be fished out again, which is 

 somewhat troublesome ; nevertheless, a syringe of this kind is 

 often serviceable in the absence of a better one. 



If the opacity is due to enclosed particles, or incrustations on the 

 membrane, recourse may be had to chemical reagents for their 

 removal. We will give further details upon this point later on. 



The spreading out of preparations upon the slide is usually 

 effected by a slight pressure with the needle upon the cover-glass. 

 This simple process affords the advantage of alternately increasing 

 and diminishing the pressure at will, according to circumstances, 

 and is otherwise serviceable for spreading out the object favour- 

 ably for observation. If, further, a certain constant pressure is 

 required to be maintained for some length of time, either for 

 conveniently examining a large surface, or for making a drawing 

 with a camera lucida, a mechanical contrivance, which admits of 

 pressure by means of screws, is useful. Such contrivances the 

 so-called compressors are now supplied by most opticians at a 

 moderate cost. 



For the observation of living cells in normal vegetation, or under 



