PRACTICAL EXERCISES 25 



1. The slides thus prepared cannot be kept, since 

 the water would evaporate. 



2. The refractive index of water being relatively low, 

 the objects do not appear so transparent as in more 

 highly refractive media. 



3. Bubbles of air are very apt to be included with 

 the object. 



I. Take the most mealy-fleshed Apple that is to be 

 had (a mellow American Baldwin pippin will do well), 

 split it in half, and with a scalpel remove a very small 

 portion of the pulp near to the core : place it in a single 

 drop of water on a glass slide, and tease it out gently 

 with needles, so as to separate its constituent parts : 

 cover it with a cover-slip (p. 14), and examine under 

 a low power (1 inch) : focus carefully (p. 18), and 

 observe 



1. Numerous, more or less irregularly oval, colourless 

 sacs, lying sometimes isolated, sometimes associated 

 together in irregular groups : these are the constituent 

 cells of which the pulp is composed : having been ex- 

 posed to no treatment which would injure them beyond 

 the mechanical disturbance of teasing, most of them 

 will be alive, and will maintain their oval form. 



Select one such, adjust the slide so that the cell 

 shall be in the middle of the field, put on the high 

 power (\ or \ inch : see pp. 18, 19), focus carefully, 

 examine it in detail : note 



2. That it is limited by a thin, smooth, continuous 

 membrane the cell- wall. 



3. That in close apposition to this internally is a 

 film of transparent granular substance : this is the 



