DETERMINATION OF THE MAGNIFYING POWEK. 227 



H 



such squares contained in it. This mea- 

 l surement, which is rarely used, except to 

 astonish the pubhc, is called the super- 

 ficial magnifying power, and is found 

 by squaring the linear, or, in other 

 Tvords, by multiplying the linear by 

 itself. The magnifying power of single 

 lenses, like the value of the divisions in 

 the different kinds of micrometer, may 



Fig. 148. 

 be set down in a tabular form, thus : — 



To ascertain the magnifying power of the compound micro- 

 scope, the method employed is the same as that proposed by 

 Hooke, in his Micrographia, as long ago as 1667, and before 

 described at page 4 ; it is thus accomplished : — Place on the 

 stage of the microscope a micrometer either of glass, ivory, or 

 mother-of-pearl, divided to some fraction of an inch ( j^ ^ will 

 answer the purpose for' all the lower powers), and, at ten inches 

 distant from the eye, hold a rule, divided into tenths of an inch, 

 in a line parallel with the micrometer ; a magnifier and eye- 

 piece being adapted to the compound body, the lines on the 

 stage micrometer are to be brought into focus; if now the 

 eye not employed in looking through the eye-piece be cast 

 as it were down on the rule, the micrometer divisions, and 

 15* 



