the telescope ; of course what is known as its ' aperture ' 

 is simply estimated by the diameter of the object glass. 

 The definition of * aperture ' in its legitimate sense of 

 ' opening ' is shown by Abbe to be obtained when we 

 compare the diameter of the pencil emergent from the 

 objective with the focal length of that objective." 



" Thus we see that, just as in the telescope, the 

 absolute diameter of the object glass defines the 

 aperture, so in the microscope, the ratio between the 

 utilized diameter of the back lens and the focal length 

 of the objective defines its aperture. This definition is 

 clearly a definition of aperture in its primary and only 

 legitimate meaning of an opening, that is, the capacity 

 of the objective for admitting rays from the object and 

 transmitting them to the image." 



" Hence ' aperture ' means the greater or less 

 capacity of objectives for gathering in rays from 

 luminous objects." 



Many objectives are made in which the rear lens 

 is larger in diameter than the beam of rays which, 

 coining from an object, can be transmitted through it. 

 This, while not particularly detrimental, has no value 

 and perhaps leads to a wrong conclusion in reference 

 to angular aperture, when this is measured, as the 

 excess of image forming rays, called stray rays, may 

 indicate a greater angle and resolving power than the 

 objective really possesses. 



73 



