SOME EXPERIMENTS ON THE ZONAL ABERRATION OF LENSES. 753 



annulus of light received on the screen from the outer zone is distorted : 

 the lowest part being displaced upwards. If the angle of tilt had been 

 smail (say about 10°) the form of the annulns would have resembled 

 Fig. 5 (1). With a tilt of 20°, it would have resembled Fig. 5 (2). But 



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Fig. 5. 



with the lens tilted to 35°, as in the séries of Fig 4, the lower part of 

 the zone has, even in the first picture of the séries, Fig 4 (1), actually 

 crossed the npper part of the annulus, and has the form shown in Fig. 

 5 (3). In Fig. 4 (2), the screen has been removed 8 centimètres further 

 froin the lens, with the resuit that the diameter of each annulus is reduced, 

 and the distortion of the- lower part has given to it the twisted form 

 sketched separately in Fig. 5 (4). As the screen is successively removed 

 backward from the lens the form distortion produces the form Fig. 5(5); 

 which, as the screen is still further removed changes into the ovate 

 form Fig. 5(6), in which the highest part corresponds to the bottom of 

 the zone of the lens, and the lowest part to the uppermost part of the 

 zone of the lens. It will also be seen on examination of the séries of Fig. 

 4, that a similar set of changes occurs for each of the other zones of 

 the lens; but that (owing to the lens not being corrected for central 

 aberration) thèse charges occur at greater distances for the inner zones 

 than for the outer zones. For instance in Fig. 4(2) the annulus of the 

 outermost zone is distorted so as to resemble Fig. 5(4) while the annulus 

 of the next bright zone is only distorted so as to resemble Fig. 5(2); 

 and the annulus of the innermost zone only resembles Fig. 5(1). In 

 Fig. 4(3) the innermost (or third) zone is in a stage of distortion such 

 that the lower half of the annulus has been displaced upwards so that 

 it is almost superposed upon the upper half; and both have been flatte- 

 red so as to présent the form of a horizontal line slightly convex up- 

 wards. This is the nearest approach to a focal line presented by the 

 bundle of rays of this zone at any part of its path. 



Experiment 3. By varying the distance of the lens from the lamp, 



ARCHIVES NKEKLANI) AISES, SÉRIE II. TOME VI. 8 



