S MARVELS OF POND-IJFE. 



to give a little general information, special pieces 

 of apparatus to use with a microscope will not be 

 described, but the most necessary will be introduced 

 in the following chapters, and the mode of using 

 them explained. 



Beginners will be glad to know how to obtain 

 the magnifying power which different objects re- 

 quire, and it may be stated that with a full-sized 

 microscope, a two-inch object-glass magnifies about 

 twenty -five diameters with the lowest eye-piece; a 

 one-inch object-glass, or two-thirds, from fifty to 

 sixty diameters; a half-inch about one hundred; a 

 quarter-inch about two hundred. The use of higher 

 eye-pieces adds very considerably to the power, but 

 in proportions which differ with different makers. 

 The instrument used by the writer has three eye- 

 pieces, giving with a two-thirds object-glass, powers 

 of sixty, one hundred and five, and one hundred 

 and eighty respectively; and with a fifth two hun- 

 dred and forty, four hundred and thirty, and seven 

 hundred and twenty, which can be augmented by 

 the use of the draw tube. When an instrument 

 is bought, the optician should be requested to give 

 a table of the action of its object-glasses and eye- 

 pieces, and there will then be no difficulty in 

 selecting the combination necessary to follow the 

 observations of others. When it is desired to 

 examine delicate surface markings of extreme mi- 



