950 



Dr. Gr. Johnstone Stoney on 



diagram, become singularly distinct when the iris diaphragm 

 under the condenser is sufficiently closed. If we arm the 



The parts of the indicator diagram made visible by dry objectives *. 



microscope in succession with different dry objectives, and 

 look at the contents of their concentration images produced 

 by this diagram, we shall see how much more of those 

 details become visible with an objective of large than with an 

 objective of small NA, in other words how much more o£ 

 the complete indicator diagram the objective of large aperture 

 makes visible. 



(b) When we employ oil-immersion objectives n, which is 

 the ratio of the index of refraction of the oil under the 

 objective to that of the air above it, is 1*515. This therefore 

 is the n which we are to employ in our figures as the radius 

 of the indicator diagram, in order that on the same scale NA 

 may represent the radius of the concentration image. Hence 

 if we are employing an oil-immersion objective of ISA. = 1*35, 

 the outer circle of the annexed figure will be the boundary 

 of the complete indicator diagram and on the same scale 

 the next circle will be the part of it which is visible on 

 looking down the tube towards the back lens of the objective. 

 Each optical punctum of this image x is the concentrated 

 light of one of the small conical sheafs of ufw's, into which 

 the light which has been transmitted by the objective might 



* In this diagram the outer circle represents the entire indicator 

 diagram. The circle Cc is the part of it made visible by an objective of 

 NA=095 ; the circle Dc?is the visible part when the NA of the objective 

 is 065" ; and the circle E<? is the part made visible bv an objective of 

 NA=0-17. 



The dotted circle is the disk of light which each punctum of the Hanie 

 can furnish when a condenser of net = 0-9 is accurately adjusted. 



