MEASURING APERTURES. 57 



Dr. Blackham's lens is of crown glass, with a thickness 

 of 0-33 inch, and a radius of curvature of 0-45 inch. 



The swinging substage bar is made to carry a small 

 candlestick, in which is placed a candle, so that when the 

 body of the microscope is horizontal, the light can be swung 

 round the object as a centre until either the image of the 

 object is imperfect or the centre of the field darkened. 



In practice, the object (with the plano-convex lens 

 attached to the under surface of the slide) is laid upon the 

 stage and exactly focussed, the best definition being 

 obtained by use of the collar adjustment. If now the 

 angle that the most oblique ray makes with the optic axis 

 be measured, the angle of deviation in glass is obtained^ 

 from which the numerical aperture can be obtained by 

 simple inspection from the table given in Chapter V., and 

 the air or water angle quite as readily estimated. 



Professor Abbe's apertometer will be described in 

 Chapter IX. 



