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IV. — On the Aperture of Object-glasses. By F. H, Wenham. 



The number of degrees that include a pencil of light radiating 

 from one single point from the focus of a microscope object-glass, 

 represents the true angle of aperture. 



In order to obtain accurate results, the light indicating the 

 admitted degrees should be confined to a mere point or line situated 

 in that focus. 



In the diagram the angles are the result of ascertained dimensions 

 from a kno^vn immersion object-glass, as follows : — Acting diameter 

 of front lens, • 033 ; focal distance from the front surface, * 025, 

 ■with a width of field of • 020, or ijj inch. With correction adjusted^ 

 the resulting angle from central focal point is 66i-^, each ray giving 

 a distinct image. In order to determine the direction of the outer 

 rays by the usual sector method, a narrow slit must be set in the focus : 

 the extreme light of the true aperture will pass through from a. 



Let the slit now be opened out to ^^ of an inch, so as to admit 

 the eflective obhque pencils embracing the entire field of view. 

 Light will then enter from direction h. This ray will also show a 

 distinct image, and an aperture of 93^ will now be indicated instead 



