\_CH. I 



MICROSCOPE AND ACCESSORIES 



that this is the sine of 31 45^' whence 2 u or the entire angle (balsam 

 or oil angle) is 63 31'. 



That is, three objectives of equal resolving powers, each with a 

 numerical aperture of 0.80 would have an angular aperture of 106 16' 

 in air, 73 58' in water and 63 31' in homogeneous immersion liquid. 



For the apparatus and method of determining aperture, see 

 appendix. 



33. Table of a Group of Objectives with the Numerical 

 Aperture (N. A) and the method of obtaining it. Half the angular 

 aperture is designated by u and the index of refraction of the medium in 

 front of the objective by n. For dry objectives this is air and n = i, fot 

 water immersions n = f.jj, and for homogeneous immersions n = 1. 

 (For a table of natural sines, see third page of cover. ) 



