CHAPTER II. 



WHAT IS ANGULAR APERTURE. 



During the lengthy controversy which occured be- 

 tween Mr. F. A. Wenham. optician to the firm of Ross 

 and Co., of London, and Mr. R. B. Tolles, of Boston, 

 Mass., the originator of the celebrated duplex object 

 glasses, the former presented ideas as to the functions 

 and nature of angular aperture which were not in strict 

 accordance with the popular views. Since then others 

 have done likewise, and it may be well enough to leave 

 the question open, without attempting to answer the 

 above interrogatory at all. 



For the information of those, however, who are just 

 entering the study of microscopy, we will undertake to 

 tell what angular aperture was, or to be more definite, 

 what it was in 1856 ; and for this purpose, the author 

 selects the definition found in the Micrographic Diction- 

 ary by Messrs. Griffith and Henfrey, a work generally 

 acknowledged as authority in matters microscopical. 



The angular aperture of an object-glass is the angle 

 measured by the arc of a circle, the centre of which is 

 formed by the focal point of the object-glass, the radii 

 being formed by the most extreme lateral rays which 

 the object-glass will admit. 



Thus let L, in the left-hand figure 1 below, repre- 

 sent the lower portion of a microscope, objective, 



