3(54 Transactions of the Society. 



advantage of the method lies in the fact that the readings can be 

 obtained without removing the slide, or even changing the focus of 

 the objective. For measuring aplanatic aperture, etc., precisely the 

 same process is used as that described. 



When dealing with the angle of diffraction of a beam of light 

 by an object on the stage, it was found advantageous either to make 

 the angle of incidence equal to the angle of emergence, or to make 

 it normal. For by so doing the subsequent calculation of the 

 fineness of the structure was found to be rendered far more easy. 

 In making the latter observations it was found better to measure 

 both edges of the beams, and take a mean, than to attempt to 

 measure from their centres. 



A method somewhat similar to the above, but involving the 

 use of an additional lens system, and higher ultimate magnifica- 

 tion, with the attendant loss of light, has been described by 

 Gordon.* Its great disadvantage lies in the fact that an exact 

 knowledge of the magnification is necessary. 



* J. W. Gordon. See June Proceedings, post. 



