Marvels of Pond-Life, 3 



glasses will not only bear long tubes, but also eye-pieces 

 of high power, without any practical diminution of the 

 accuracy of their operation, and this is a great con- 

 venience in natural history investigations. To obtain 

 it, however, requires such perfection of workmanship 

 as to be incompatible with cheapness. An experienced 

 operator will not be satisfied without having an object- 

 glass at least as high as a quarter, that will bear a deep 

 eye-piece, but beginners are seldom successful with a 

 higher power than one of half-inch focus, or there- 

 abouts, and before trying this, they should familiarise 

 themselves with the use of one with an inch focus. 



It is a popular error to suppose that enormous mag- 

 nification is always an advantage, and that a microscope 

 is valuable because it makes a flea look as big as a cat 

 or a camel. The writer has often smiled at the excla- 

 mations of casual visitors, who have been pleased with 

 his microscopic efforts to entertain them. ^' Dear me, 

 what a wonderful instrument ; it must be immensely 

 powerful ;" and so forth. These ejaculations have often 

 followed the use of a low power, and their authors have 

 been astonished at receiving the explanation that the 

 best microscope is that which will show the most with 

 the least magnification, and that accuracy of defi- 

 nition, not mere increase of bulk, is the great thing 

 needful. 



Scientific men always compute the apparent enlarge- 

 ment of the object by one dimension only. Thus, 

 supposing an object one hundredth of an inch square 

 were magnified so as to appear one inch square, it 

 would, in scientific parlance, be magnified to " one hun- 



