588 Dc. Gr. Johnstone Stoney on 



geometrical properties of that image*, as given in sections 

 33 and 34 of the paper on " Microscopical Vision " in the 

 Phil. Mag. for December 1896, with those of the reference 



Fig. 12. 





/tianc 



Reference-hemisphere, of which the circular base is the indicator 

 diagram. 



hemisphere and indicator diagram, as given in sections 7-10 

 of the paper in the Phil. Mag. of February 1903. From 

 these geometrical properties we can deduce the rules to be 

 observed in making experiments. 



In all microscopes the medium above the objective into 

 which the light emerges, is air ; but the medium under the 

 objective from which the light is supplied to it differs in 

 different cases. It is air when we employ what are called 

 " dry " objectives, and it is water or oil with " immersion " 

 objectives. Lagrange's formula — viz. : 



n sin a = M sin /3, 

 where M is the number of times that the linear dimensions 



* In the paper here referred to the concentration image is called 

 image x. 



