380 NOTES. 



stage micrometer in terms of the eye-piece micrometer, are called 

 M, so we will continue to represent them by that letter. 



The determination of the combined powers of a battery of 

 •object-glasses with various eye-pieces is a tedious business. 

 There are various ways in which it may be done ; one is by 

 projection in a photomi orographic camera ; another, and more 

 usual plan, is by means of a simple form of camera, such as a 

 WoUaston's camera or Beale's neutral tint, to cast down the 

 image upon a divided ruler ; these methods are so well known 

 that further description is unnecessary. The new plan is as 

 follows : After having found the value of M for each objective 

 in the battery, one object-glass is selected (preferably one of 

 medium power, say 1/2-inch), and by one or other of the above 

 methods the combined magnifying power of this object-glass with 

 All the eye-pieces is carefully measured. These measurements 

 are then divided by the M value for that object-glass. This 

 gives a constant for each eye-piece. The combined power of 

 any other object-glass with any eye-piece whose constant is 

 known is found by multiplying the constant of the eye-piece by 

 its M value. 



I have just arranged a battery of twelve object-glasses and 

 four eye-pieces with a microscope for work on pond life. The M 

 values of the object-glasses were carefully measured, the com- 

 bined power of one object-glass, the 1/2-inch, was also measured, 

 these data gave the constants of the eye-pieces, then the com- 

 bined powers of the remaining eleven object-glasses with the four 

 «ye-pieces were read ofi a slide rule, at sight, without any trouble 

 at all. Example : 



Here 103 di\dded by 11-75 is equal to 8 '77, the constant of No. 1 

 «ye-piece ; this constant 8-77 multiplied by 33-33 gives 290, the 

 power of the l/6th with No. 1 eye-piece, and so on. 



Perhaps some would prefer the reciprocal method ; for neces- 

 sarily the result is the same whether a number is multiplied by 

 another or divided by its reciprocal. The following example will 

 explain. 



