PREFACE. XV11 



will, when Nature is more closely and affection- 

 ately studied, be lost, like us, in wonder, love, 

 and praise. 



Microscopists usually speak of the power of 

 their glasses in diameters, or what is termed, 

 also, linear dimensions; that is, the simple 

 measure across, instead of the compound or 

 superficial measurement : a power of 250 dia- 

 meters (quite sufficient for all ordinary purposes) 

 has a superficial measurement of 62,500. The 

 size of objects is calculated by parts of an inch ; 

 an animalcule may, therefore, be TO, TT&-, or 

 even 12000 of an inch in size : this may be 

 ascertained by knowing the diameter of the 



