198 



Transactions of the 



III. — An Instrument for excluding Extraneous Bays, m 

 measuring! Apertures of Microscope Object-glasses. 



By F. H. Wenham, Vice-President K.M.S. 



{Read before the Eoyal Microscopical Society, April 1, 1874.) 



The angle of aperture of an object-glass is taken from the focal 

 point through which the rays must pass for all degrees. Those 

 entering at other angles within the plane of focus will give a false 

 indication, from diffused light forming no image. 



If a conical nozzle, having a small aperture in its apex, is 

 placed over the front of an object-glass, the height of the cone 

 being equal to the focal length, all such false light will be excluded, 

 and image-forming rays only will enter at angles of any extent. 



As there would be a difficulty in adapting such cones to every 

 object-glass to be measured, an instrument has been contrived to 

 meet the requirements. The traverse is horizontal during the 

 measurement ; therefore a vertical slit will serve instead of a 

 circular stop. For high powers it is requisite that the metal 

 edges of this slit should be exceedingly thin, and consequently must 

 be secured from damage from contact with the object-glass or 

 otherwise. The outer sides are required to be exactly in the focal 

 plane during the measurement. 



The cuts, full size in plan and side views, illustrate the arrange- 

 ment, a a is a plate of brass with a central square opening 

 chamfered away beneath so as to clear 170^ ; h is a slip of glass 



