Dr. G. J. Stoney on Microscopic Vision. 



509 



We should therefore put in a stop 

 which will block out all beams that 

 have a grasp less than this *. The 

 image at x will then have the ap- 

 pearance represented on a large scale 

 in the annexed diagram. An annulus 

 of bright light will be seen in it 

 between the edge of the stop, which 

 lies at ^ / = 0'71, and the limit of 

 the grasp of the condenser at G^O'85, 

 and faint light will be seen farther out to the limit of the 

 grasp of the objective at G = l*25. 



We may, however, with advantage block out still more of 

 the light. For the specks upon the diatom are numerous, 

 and although not quite regularly disposed, they are approxi- 

 mately in straight vertical rows, and less regularly in some- 

 what wavy horizontal rows. In this description we suppose 



Image x, with X scale. 



A 







o 



Z>o 



z 



o 



Oct 







d 



o 



* To get the stop accurately into its place it should be mounted 

 so that it can be centred relatively to the condenser. Provision for 

 this adjustment is always useful and sometimes essential for accurate 

 work ; and Messrs. Watson and Sons, at the request of the writer, made 

 an adjustable cell for holding the stops which is entirely satisfactory. 



