SOME EXPERIMENTA ON THE ZONAL ABERRATION OE LENSES. 755 



Experiment 1. À white screen was placed about 3 mètres away to 

 receive the balloon-shaped patch of light. A wire or straight rod is 

 then held horizontally across tbe beam at a place about 15 centimètres 

 from the centre of the lens and a little above the level of that centre. 

 It casts upon the screen as its shadow an entirely detached ring. The 

 reason for tins surprising phenomenon is that in the non-homocentric 





OB JECT 



SHADOW 



A 



A 





0 



Jd 





t 



p 





UU 



D 











E 





8 



Fig. 6. 



pencil it intercepts. when so held, the light from a (non-concentric) 

 zone of the lens, and therefore yields on the reversed image of that 

 zone in the coma on the screen a ring shadow. On lowering the level 



48* 



