300 E. M. NELSON ON INITIAL MAGNIFYING POWERS OF OBJECTIVES. 



It will be noticed that Mr. Nelson lays stress on what he has 

 named the Optical Index ; but perhaps it is less apparent that 

 this paper on the magnifying power of objectives, and his com- 

 munication to the last meeting on their aperture, are quite 

 closely related to the Optical Index in fact, they deal with both 

 the values involved in the formula for the Optical Index of an 



, . .. Numerical Aperture x 1,000 



objective = A ^ . 



Magnifying .rower 



A general way of expressing the meaning of the Optical Index 



of an objective is that it is the ratio of its aperture to its 



power. J. Grundy.] 



Joura. Quekett Microscopical Club, Ser. 2, Vol. XII., No. 74, April 1914. 



