178 TESTING THE MICROSCOPE. 



VII. 



DETERMINATION OF THE MAGNIFYING POWER AND 

 FOCAL LENGTH. 



IT is customary to estimate the power of a Microscope by the total 

 magnification, but that of objectives by their focal lengths. How- 

 ever little the former may be a safe criterion, since it is deter- 

 mined by factors of very unequal values, yet a knowledge of it 

 is indispensable even from a practical point of view. The focal 

 length, on the contrary, stands approximately in inverse ratio to 

 the objective amplification ; it may to a certain extent be regarded 

 as an expression of the magnification not dependent upon the tube- 

 length, and for the comparison of different objectives furnishes, 

 therefore, the requisite standard. The determination of the focal 

 length, as well as of the total magnifying power, is therefore 

 essential in a complete testing of a Microscope. 



1. MAGNIFYING POWER. 



We have already shown that the magnifying power m is pro- 

 portional to the distance of the virtual image from the second 

 focal point, or, what is nearly equivalent, from the eye-point of 

 the Microscope, so that twice the distance corresponds to double 

 the magnification. If, therefore, the magnifying powers are to be 

 comparable inter se, they must logically be referred to equal 

 distances. The distance chosen is entirely immaterial to the 

 comparison. In recent times it has been customary to calculate 

 the magnifying power for a distance of 25 cm., or 10 inches. 



The so-called "curvature of the field of view" involves the 

 fact that the amplification of the marginal parts is usually some- 

 what greater than that of the central ones ; hence an accurate 

 determination of the magnification is only possible for a relatively 

 small part of the field of view ; from well-known practical con- 

 siderations it is, as a rule, determined only for the centre, omitting 

 a tolerably broad peripheral zone. 



